pic: Dual FP gearbox

Those distances added to the center to centers all depend on your plate material, stiffness, machining capabilities, and so on. The team I've worked with has always been fortunate enough to work with a machine shop that could hold true to .0001, so we never really saw a need to design around the machining equipment.

What will these be machined on? If you find what tolerances they hold, it'll be easier to identify what spacing you need.