trying to find the percentage increase/decrease using a formula

I'm trying to formulate a formula to calculate the percentage increase/decrease. I know how to calculate it manually but am not sure how to put it in a formula.

Ex:
G36 = a formula that calculates the sum of numbers, say 1000.
G37 = a formula that calculates the sum of numbers from a previous year, say 500.
I need H37 to calculate the percentage difference of those two.
Manually, it would be 1000 + 500 = 1500 -1000 / 1000 = .5

Using basic logic, not Excel logic, gives me this:

H37 = (g36+g37-g36)/g36 gives me .32

I feel I need to use something similar to this formula:

=(E12-(E12/3))/3 this is formula I use in my sheet that works using itself for another task, though it's only called twice while the other requires 3 instances.

Thanks
Mike

Update: Never mind, that formula I have does work. My totals were adjusted by mistake during testing, making my percentage look like it was wrong, when in fact it was correct for the given totals.