Routing speed?

Building a Mill.
Shopping around for lead screws.
What would be the ideal routing speed for cuting out circuit boards.
The motor I have run verily smooth at at or over 200 rpms 1/8 step. At
5 tpi would put me at 40 inches per minutes. Is this too fast, too slow?
I have no idea as to what speed to look for.
Most of the lead screws I see for sale is at 5 or 6 tpi.
I want to drive the lead screw direct with 269oz stepper motors.
Sure need some advice on this before shelling out the bucks here.
thanks
bill