Step 17: Selecting BB weight.

This is a little trick mentioned by Star_folder over at AIrsoft Forum:

Take your FPS ( with .2s) and...
...divide by 1600. This is the lightest BB you should use.
...divide by 1200. This is the heaviest BB you should use.
...divide by 1400 for a good compromise.

In reality, the maximum BB weight you can use is more a function of your hopup, but this is generally a nice estimate.