I'm not daring enough to use the EFI shell yet. ;-)
		
		
	 
but it's fun. If you have RefindPlus, might as well look at the EFI Shell.
First thing you do is type 
mode to get a list of text modes.
Then type 
mode x y to change the mode to the one with the most vertical lines.
Then type 
help for a list of commands.
Then type 
map for a list of file systems and block devices.
Then type 
fs0: to chose the first file system (like C drive of Windows)
Then type 
ls to get a list of files on that file system.
If it has an EFI folder, then it's probably an EFI partition and you can use mkdir to make a directory and cd to change directory, and you can do commands and pipe their output to a file
dmpstore > dmpstore.txt
Then you can boot into macOS, mount the EFI partition, and view the file you created.
	
		
	
	
		
		
			Anyway, not sure what you are suggesting to discover with that?
		
		
	 
The point is to gather info about your system so you can tell if Windows changes anything important afterward. You can discover what nvram variables it changes, or what boot code it adds to the MBR or PBR.
	
		
	
	
		
		
			Dayo has some excellent instructions on that other thread about installing Windows using a VM first to run the Windows installer...  the only thing is his instructions involve using a whole drive, rather then a partition on a drive.  Which doesn't have to be a big deal, but I read some comments on this forum about people who setup dual boot systems and then at some point a windows updater may have written outside the partition into a boot sector of some and kind of screwed things up.  I don't really remember the details of what I read now, but I am just wanting to make sure I don't run into any kind of weird problem like that.  
I guess at this point all the drives in my box were partitioned with GUID.  So if I have to somehow use MBR, then again, I don't know how I would do that as a FAT partition on one of these, might be better to just buy another drive and format it purely the windows way with MBR and FAT on top.  Yea?
		
		
	 
A VM is one way to do it. You still want to gather the info about your system before doing the install. So you have something to compare when it's done.
You don't need to change to MBR. You keep GPT and make hybrid MBR/GPT which is the same as GPT except the first block contains boot code to do BIOS boot of Windows, and the first block has a MBR partition table with up to 3 partitions specified (first partition should still by type EE - for EFI but is changed to extend only to the start of the first partition). iPartition.app does this with a couple clicks. Use the dumpvols.sh script before and after you do iPartition, to learn what iPartition does to your disk. iPartion.app cannot resize APFS partition so leave them alone. If you need to resize an APFS partition to create the FAT partition, then use Disk Utility.app to create the new partition. Then use iPartition.app to make the new partition Active and Visible in Windows.
	
		
	
	
		
		
			You can't do MBR with FAT. It is either/or. The Apple way is a hybrid partition when installed on the same drive (at least on older macs). When a hybrid parturition is created Windows cannot see the EFI partition. As far as it is concerned it is an MBR drive (even though it is not hence it is called hybrid).
		
		
	 
It's a FAT partition on a hybrid GPT/MBR disk. macOS treats it as GPT. Windows will treat it as MBR. Don't do partitioning in Windows. Only use Windows to erase the partition as NTFS (you can tell what partition is the correct one by looking at the size and name).
	
		
	
	
		
		
			As mentioned above I'm having issues with the RefindPlus resolution. It looks like it might be 1024x768 or 1280x1024 scaled to fit 16:9 (so stretched laterally/horizontally - optically it all looks fat and wide). Photo attached.
Below is a (partial) RefindPlus log that includes all the info about Graphics, GOP and resolution.
	
	
	
		Code:
	
	
		15:172   0:023  Read Config...
 15:198   0:025  Detected Overrides - Read Config...
 15:250   0:051  Adjust Default Selection...
 15:271   0:021  Initialise Screen...
 15:297   0:025  Check for Graphics:
 15:322   0:024    - Seek Console Control
 15:368   0:046      * Seek on ConsoleOut Handle ...Success
 15:394   0:025    - Assess Console Control ...ok
 15:418   0:024    - Seek Universal Graphics Adapter
 15:443   0:024      * Seek on ConsoleOut Handle ...Unsupported
 15:469   0:025      * Seek on Handle Buffer ...Success
 15:493   0:024      ** Examine Handle[00] ...Success
 15:518   0:024      *** Select Handle[00] @ 640x480
 15:543   0:024    - Assess Universal Graphics Adapter ...ok
 15:569   0:025    - Seek Graphics Output Protocol
 15:618   0:049      * Seek on ConsoleOut Handle ...Success
 15:644   0:025    - Assess Graphics Output Protocol ...NOT OK!
 15:668   0:024  Validate Replacement GOP for ConsoleOut Handle:
 15:693   0:024    - Seeking Firmware GOP Handles ...Success
 15:744   0:050    - Found Candidate Replacement GOP on Firmware Handle[01]
 15:768   0:024      * Evaluating Candidate
 15:793   0:024      ** Valid Candidate : Width = 1280, Height = 1024
 15:843   0:050  INFO: Provide GOP on ConsoleOut Handle ...Success
 15:893   0:049  Query GOP Modes (Modes=5, FrameBufferBase=80000000, FrameBufferSize=0x0):
 15:919   0:025    - Mode[00] ...Success @  1280x1024  ( 1280 Pixels Per Scanned Line, 8bit BGR Pixel Format )
 15:968   0:049    - Mode[01] ...Success @   640x480   (  640 Pixels Per Scanned Line, 8bit BGR Pixel Format )
 16:018   0:050    - Mode[02] ...Success @   800x600   (  832 Pixels Per Scanned Line, 8bit BGR Pixel Format )
 16:068   0:049    - Mode[03] ...Success @  1024x768   ( 1024 Pixels Per Scanned Line, 8bit BGR Pixel Format )
 16:151   0:083    - Mode[04] ...Success @  1280x960   ( 1280 Pixels Per Scanned Line, 8bit BGR Pixel Format )
 16:205   0:053  Set Screen Resolution:
 16:229   0:024    - BestMode: GOP Mode[0] @ 1280x1024
 16:261   0:031    - Switch to GOP Mode[0] ...Success
 16:285   0:024  Screen Resolution Set
 16:310   0:024  INFO: Implemented Graphics Output Protocol
 16:341   0:030  INFO: Implement Text Renderer ...Success
 16:363   0:022  Setup Screen...
 16:410   0:046  Prepare for Graphics Mode Switch:
 16:434   0:024    - HiDPI Not Detected ...Maintain Icon Scale
 16:460   0:025  INFO: Running Graphics Mode Switch
 16:485   0:024  Refresh Screen:
 16:510   0:024    - Get Banner
 16:590   0:079    - Scale Banner
 16:643   0:053    - Clear Screen
 16:675   0:031    - Show Banner
 16:709   0:033  INFO: Switched to Graphics Mode
	 
 
My GPU is a Sapphire Pulse RX590 8GB - it has a single BIOS (so doesn't have a bios switch). I connect to a Samsung U28E590 28" 4K display via DisplayPort cable with DisplayPort 1.2 setting and with Freesync on. It has a pixel density of 157PPI. I get the same result regardless of which DisplayPort I connect from on the back of the GPU (it has 2x DP). 
Does anyone know what I need to change/set in order to get the ratio right for my setup?
		
 
		
	 
Strange that you're not getting any 16:9 modes. Maybe the Radeon EFI driver sucks. But at least you're getting an image. Unless it's using the wrong GOP or UGA on GOP or something weird. In EFI Shell (access from RefindPlus), you can do this command (after setting the current directory to a writable directory as explained above.
dh > dh.txt
dh -d -v > dh_d_v.txt
Edit: script moved to 
https://gist.github.com/joevt/a99e3af71343d8242e0078ab4af39b6c