Not sure about your formula but best answer is: variables will change so any math will as well. I'd use CG's tool to help but with that kind of hashing you'll know after a few days where your derp will lay for this coming block.
Ive been mining on this pool for if I had to guess around 2 weeks, and have renting a lot of hashing power so I get to full 5nd as fast as possible. Thats why im trying to figure out what my full payout will be to see if I have reached 5nd. Not sure how to calculate when you throw rentals into the equation.
Just a heads up. The numbers don't add up for renting hashpower just to make up the 5ND so I recommend against it unless you're getting it for very cheap (which you're usually not.)