It's more important to get the max current right and not worry too much about the voltage. The higher voltage gets the motor current up quicker on each pulse which helps keep the torque up. I'm running 3Nm motors on 68V with no problems, like a lot of people. I'm running 2 X axis motors, 3Nm, each driving 2005 ballscrews. I'm getting critical speed on the ballscrews, very similar size to yours, which is around 5m/min rapids. My gantry is probably heavier than yours as well. When you do the sums, don't forget the inertia of the ballscrew as well - when you get to this size, that value can be as much as the gantry.

Clive'a rule of thumb advice is on the money - do the sums, analyse as much as you like, and the answer's pretty well always 3Nm (4Nm), 68V! No point in going smaller (doesn't save much money anyway) and bigger gives worse performance.