I think that "setting the properties directly" for the % is a very important help, because actually when I'm designing a new layout I think on absolute values instead of % values.
This could improve the productivity of layout design.
Actually :
1° step : design in absolute values
2° step : coding the % values
Pure designer % settings :
I do only the first step.
Please (Erel / Klaus) consider this point in the next release.