I don't have a formula, per se, but I used to have a system whereby anything the players did to advance the play of the game was rewarded with points, though not very many. However, little awards like that can stack up. That way the guy who came up with the idea for doing something still gets some credit for it even if he isn't there when it gets done. I also awarded points to people who used their skills, took advantage of opportunities, etc. Anything they did other than just sitting around waiting for something to happen was rewarded. This required making a list at the end of every session of what was done and by who-that could be a bit cumbersome.