We have typically used a 0.08 mm increase in gear center distance with our custom-made geartrains. My guess is that this is on the high side compared to some others who make their own boxes, but these are typically wire-edm'd gears whose surface finish is a little rough before run-in. Our gears are not very wide either.
We run them in at no load (i.e., not driving the chassis around, but just running the wheel while it is off the ground) for an hour or so. We have been using the same tube of Amsoil grease (apparently its "racin' grease"
) for several years now....
If I was using purchased, nicely finished, wider gears, I think something in the 0.05 mm range would be my choice for the amount that the center distance is increased.
The looser distances like I am suggesting here make the boxes easier to assemble and get low geartrain friction, in my opinion.
Ken