How to set UV grid resolution

How do I set the grid resolution in the UV/Image editor to 64x64 please?

I want to create lightmaps for modular surfaces in UE4, and the engine bleeds something shocking if things aren’t aligned properly to the grid.

The only workaround I can imagine is making a 64x64 image in GIMP and then snapping to that in Blender, but this is unintuitive. The grid lines take up 1 pixel, so I don’t know whether to start off with a grid line at 0.0 or whether to miss out the first line.

Any advice is appreciated.