From Polycount wiki: "
Visual Packing Guide
If you want a visual guide when creating your UVs, one easy way to see the spacing is to use a checker as the background image in your UV editor, with each checker square set to the padding width.
For example to get a 4-pixel-wide checker on a 512x512 image, tile the checker image 64 times... if you calculate that the checker image has 2 checker squares across it, then the checkers bitmap should be 8 pixels across for a 512 image, and 8 pixels goes 64 times into 512, so that means it needs to be tiled 64 times across your UV square. Remember though, that if your UV space isn't square, then the checker tiling needs to be adjusted."