You can approximate the breakaway torque by considering static friction only. From the static friction coefficient of the lead screw (by knowing the maximum load on the spindle, coming from the application) you can calculate the static friction torque of the nut on the lead screw.
You should do the same with all linear guides and/or bearings that you have and sum them up. That will be your breakaway torque.
For the lead screw: F = c*N where F is the static friction force, c is the static friction constant and N is the normal force. In this case you have to convert the Force to Torque Torque = Force x Lead /2pi. Or the lead screw manufacturer might give directly the constant which links static friction constant to torque.
