This is actually a fairly complicated question that you are asking. You're making a leadscrew.
You can probably get a first level approximation treating the thing as a ball screw with a very very low efficiency (50%?)
Driving torque is in Newtons
Thrust Force in Newtons
Screw Lead in meters (distance the nut travels along the rod during one revolution)
Driving Torque = (Thrust Force * Screw Lead ) / ( 2 * pi * efficiency)
So for you,
Thrust Force = (Driving Torque * (2 * pi * .5) ) / Screw Lead
For a
rough approximation.
This will give you a more accurate answer:
http://en.wikipedia.org/wiki/Leadscrew