drop to terminal and do a 'df -h'
that brings back the real space available...
ALSO don't forget that if you don't clear trash, that you officially haven't freed up that space.
Don't forget:
Webbrowser Cache
User Cache
Font Cache
System Caches i.e. kext cache
System Logs
etc.. but if you aren't sure what you're deleting, rule of thumb.. DON'T 😀