I’m ok with percentages, as I’m familiar with the way Bootstrap and Foundation work (I personally use Sass with Susy). With these grid systems, I can input the number of columns, the column width, and the gutter width, and I get all the css classes automatically generated (in percentages). After that it’s easy to add the corresponding classes when looking at the psd image file simply by counting the number of columns.
On the other hand, I’m a bit put off by Unsemantic grid, as it works differently – in steps of 5 percent. It doesn’t correspond to a twelve grid system.
I guess I’ll just have to try it out and see how it works, making approximations and maybe playing with padding to get it just right.