tsuwm,

Since I know you're a stickler for using the right word, I consider it my duty to jump in here and point out that a series that goes 25,50,100,200,400 is in fact a geometric series (each number a constant ratio of the last). 700 certainly wrecked that one. An arithmetic progression is one that is formed by adding/subtracting a constant number to/from the last, eg 3,6,9,12. Your new theory, if I've understood it correctly, and an interesting one on the data so far, suggests that the algorithm is based on the number of posts required at each level: (so far 25, 50, 100, 200, 300), which you're assuming has now settled into an arithmetic series, although it started as geometric. I don't know if there's a word for hybrid series like that.