If I may, I'll pass on something that works well for me. I have broken down the threads into a small component. that is then copied multiple times. That way I can reuse the threaded section and easily adjust its length.
http://farm5.static.flickr.com/4048/4484626412_c316a75d8c_m.jpg
This wood screw was drawn to be 2" long but I can quickly change it to be any length I need. The threads are separate from the rest of the screw and the threads are made up of multiple instances of the same component. I can add or subtract instances as need to make the length adjustment.
http://img.photobucket.com/albums/v506/weekender410/PipeClamp1-1.jpg
I did the same sort of thing for the threads on this pipe clamp. I can then make an acme threaded screw any length I wish.
Edited to add this image. I was looking for it earlier to add as a reference and couldn't find it.
http://farm6.static.flickr.com/5141/5662968502_1dc00d333b.jpg
The round head screw was made in a few moments from the flat head screw and the T-bolt was made from the hex head cap screw on the far right.