Formula to calculate the cumulative percent rent for each month

[d3volnik]

OK Basically, what I am trying to do is the following

on the highlighted part all the way to the left on the attache file in trying to do formula that will calculate the cumulative percent rent for each month base on the comulative sales (highlighted in yellow).on Column AG row 10 (highlighted in blue) that is the annual PCT rent that needs to be equal to column AX(the formula for this column its already set up).

for example:

if in JAN a store sales are 320,000 and has the following breakpoits

breakpoint $ (BP$) Breakpoint% (BP%)
BP$1 BP$2 BP1% BP2%
<350,000 < 450,000 10% 12%

it means the store didnt reach the BP$, and on FEB the store sold 100,000, so the comullative sales JAN+FEB =420,000
so the calculation for % rent for FEB will be the amount greater than 350,000 but less than 450,000 multiply by BP% so it will be 420K-350K=70K then multiply by the first BP% because it didnt reach the second BP$. so 70K*10%=7K

do i make any sense?? lol

please help