I'll pass on being master, since I can't really be on frequently and reliably enough to respond to guesses at a good pace. (I also like Normal_Anomaly's suggestion of just organizing a time to play in a chatroom.) If someone else stepped up, I would play again, though. This is a fun game!

My other candidate rule was "if the last digit is divisible by 3," which almost works, except that 6 was not accepted (so it would have to have a special case.)