How much longer should the lead screw be (C-Beam)?

When putting together a lead screw C-Beam actuator, how much longer must the screw be than the actual aluminum extrusion. I assume the screw must be = (length of extrusion) + 2(end plates) + ????. How far must the screw extend past the end plates for a coupler to grab it at one end and and a lock collar on the other?

I see that both the extrusion and the screws are sold the same length, and I assume I will need to trim back the extrusion.

Thanks,

 

Thanks!!!