UV space is setup in ratios. 1:1, or 2:1, etc. The default is always 1:1. Pixels don't matter until you export a texture map. I don't think you can snap to the subgrid.
I haven't used the image ratio setting in awhile; I find it simpler to pack my map in U -1 to 1 and V 0 to 1 (so, the normal UV square plus the one next to it). Then when I'm going to export, I duplicate/merge my mesh and with origin action center, scale to 50% in U and move +0.5 in U. Done. Although now that I think about…
Such a fast answer, thank you! I'm not too sure on how to set it properly though :D If I want a grid of 64x64 pixels, I set grid spacing along U+V to 1 and then subgrid count to 64x64 on the U and V? Is this the correct way of doing it? Also, is there a way of resetting it to default after I've messed up with it? And last…