This type of formula has been used for rating, well, almost everything. With a different definition of k, it was invented for chess, and is almost universally used in backgammon. It has been applied with varying degrees of success for other games. I find it only mildly inadequate for pool and cribbage (you can claim that the margin of victory doesn't matter, if you agree ahead of time to 'per-game' stakes rather than per ball or per point.) Its performance is very disappointing indeed when applied to bridge.
First word of advice for the OP: 40 points per game is way, way, way, way too big. Your entire rating system consists of nothing but random noise. In backgammon the adjustment is 2 points for a one-point match, about 5 points for standard hour-long matches, and maybe 10 points for an all-afternoon-long match -- and even under those circumstances a player's rating may move up or down by about 100 points in a month's time by chance.
As to your question about rates of decay.... applying it to your particular game will require repeating some experiments or carefully applying some scaling factors, but I think you will find Doug Zare's answer to this same question in backgammon (
http://www.math.columbia.edu/~zare/rdraft.html) enlightening.