Somewhere around 61 is about right...but you'll need a minimum amount of completed stroke play games as well...WGT has never divulged that number.
Of course nobody knows exactly how the algorithm works, but I wouldn't be surprised that 9 hole rounds carry less weight than 18. Country Club tournaments, as their difficulty is dictated by club members, and not by WGT, may be a factor as well...but who knows...it's been brought up before and it's never been denied (As far as I know).
Joey