Back EMF is a function of a ton of things, it would be difficult for you to perdict it.
If your machine has fast rapid moves, lots of inertia, low sliding friction, and you do sudden stops from high linear speeds in multiple axis that would probably push it up the highest.
My guess, that any reasonably designed mechanics in a machine tool with moderate to good performance will easily push your bus voltage up more than 5 volts.
I'd suggest a simple active load / clamp to keep it from going above say 73 volts. You'll need a power npn /n-channel transistor, an op-amp, voltage reference, and a few resistors.