Thanks, Arvid...yes I see that I mixed units, so I will note this in my post with an edit (in case someone else reads just that post, in the future). It almost seems impossible that a little 1/8 HP motor could produce 377 pounds of thrust (after gearing)...but I guess the math is okay.
I found a nice website that explains how various screws work, and it gives some neat formulas to play with interactively:
http://www.roton.com/web/application.jsp
Oh boy...the more you learn, the more you realize you have yet to learn!
Next, I'm off to do some acceleration calculations...I wonder how quickly we should expect a 150 pound gantry to accelerate from zero to 2.5 inches per second...
Thanks,
Chris