Quick question about spacing between mini V-wheels

I'm finally taking up building the D-Bot 3D printer. I was going through the excellent build guide and some of the parts listed in there. I noticed that none of the gantries have eccentric spacers in them and they all have an oval hole to adjust the screw snugness with the rails. I though I'd modify this and add eccentric spacers there.

This brings me to my question. First, what is the spacing from the centre of the mini V-wheel sitting...

Quick question about spacing between mini V-wheels