It seems that you have wrongly commented one line in the [Optimized] section, it is now:
Run,%ScriptFile%,ImaFileTest
//_Calculate,"Ceiling(%IMGSize%+%pNumberBox3%)",ImgSize
With this second line commented, it doesn't matter what value I enter for Free Space, since that value is ignored. It is not essential with the basic project, but I cannot boot my full project containing many drivers and programs resulting with 450 MB ISO. If I uncomment that line, the ISO made with the full project boots fine (well, at least it boots fine on one test computer; I am trying now to find out how I could make it to boot on the other computers as well).
Also, the default value of "Of Target" is 100% which is too low for my full project, it should be at least 120%. I don't know how the used DirGetSize is internally working, but it seems that it doesn't take in account the slack space of files related to the size of sectors on NTFS drive. Because of that slack space, the image file should be greater than 100% of the size of %TargetDir%.