I won't take credit for this tip as it was posted here on Hobbytalk by someone else (who I can't remember)
To easily set mesh using 64 pitch gears, first determine what size pinion gear you want to use. Next select the next largest pinion gear. For example, if you want to use a 25 pinion, grab a 26 pinion. Using the larger pinion, mount it to the motor and set the mesh snug with the pinion against the spur. Tighten down the motor screws. Now remove the larger pinion and put on the smaller pinion. You should now have a good working mesh for the gears.
I've tried this trick and was surprised how easy it was to use. I'm not suggesting this is the easiest or best way to set mesh, but it did work in my case.