Tough question. I'd say your estimate is as good as any. But I think women tend to be less open about such things than guys, making their numbers seem less in places like this. Just my opinion.
I'm with milagros and drew agreeing with both of them.
That said there are TONS of "vanilla" women out there who do not know they would enjoy this fetish until properly introduced (my wife for one who is now a fantastic ticklephile). So the true numbers are unknown (Maybe Jeff could give us some membership stats?) and the potential numbers are even less well defined.