Thanks everyone for the speedy replies. I must have just been doing something wrong with SolidWorks then.
Now that I know that the WCP calculator is correct (not that there should've been much doubt, I suppose), I've created this spreadsheet with values for the center to center distances with various gears sold by WCP/VexPro:
http://www.chiefdelphi.com/media/papers/3036
Thanks again!