Great advice above. My absolute favorite 40 powder is WSF, it fills the case sufficiently to make detecting double charges easy and meters well. I've used 231, but I lean toward the other. As noted above, run a plunk test with dummy rounds to determine OAL for your chosen bullet and your gun's chamber. In the 40, 1.125 OAL has worked fine for me, although if you work your loads from the start up you can go shorter and I've seen preferences of 1.135. When I've had doubts about powder charges, I've tended to default to the powder mfg data.
Something you might find interesting re: lighter loads for the 40 is at
http://www.handgunsmag.com/2010/09/2...40lite_091806/
Have fun, be safe, and welcome to the wonderful world of reloading.