You're absolutely right. I'm not exactly sure what I was thinking with that formula ... the only difference is 1.X vs. 0.X. I think I'll change that.That formula strikes me as odd. Why (1298-900)/900? Why not just 1298/900, a simple ratio of monster xp budget to PC "xp budget"?
I feel fairly sure that it won't affect the rankings one way or another; i.e. if encounter A is harder than encounter B in one system, it still will be in the other system. It just seems like a strange number to produce to me.