How accurate does what you are making need to be? We deal with gearing center to center issues whenever we swap out the change gears on our lathes. Mostly we just say that we need some backlash but not too much, and never even think of quantifying it. At most, we stick a random scrap of paper between the teeth and call it good. I am sure some gearing setups require strict accuracy tolerances, but that would be related to speed, torque, and in all cases, as Joshua said, ultimately backlash. You can control backlash while making gears and simply make them fit, if it is not a super critical application. You can also often make the center to center distance adjustable, like what we do with our lathe change gears or by adding a cam mount for one of the gear's center.