Buy the "master calibration pack", Wilson Combat also sells a nice set of springs in a organized and labeled pouch.

When I "work up" a load for my 1911s I start with a stock spring (16#.45, 14#.38sup) run my loads in the ransom, and when I've decided on my load, if the gun won't cycle, I drop the spring weight one by one till it will. In my .38super 6" I only run a 7 or 10 lb spring,and a very large radius firing pin stop, BUT, I'm throwin a 147gr bullet at only 800fps which is way slow for a Super, let alone a 6".

Its rarely necessary to go over a 18lb spring in full size .45. Plus you can always run a shock-buff if you want.