I only have the 256gb SSD. I feel regretted and should have it upgraded to at least 512gb. I don't use a lot of space but I am constantly worrying about running out of space. At this time I have only used about 32gb. I don't expect to use more than 100gb in the near future. My question now is how much free space do I need to have in order to ensure that the retina 5k will runs smoothly or at its best?