How to calculate cumulative sum of assets decreasing monthly straight line by the same %?

Hi,
I'm an accountant, trying to build a formula that would calculate fees we are charging our client monthly based on assets as of that month. Is there a way to build ONE formula that would calculate the number for the year for me?

Assumptions:
[A] Prior year ending assets - $2,716 MM
[B] Monthly change in assets (straight line, NOT compound) -1/12th of 15% decrease every month
[C] Annual fee rate - 1.318%

I can build something like this, but this is too many steps - I just want one formula, so i can calculate different scenarios for various assumptions... the hard part for me is cumulative assets for the year that I could just multiply by the monthly rate and get to the same answer. I figured out that cumulative change for the year equals MONTHLY CHANGE*78 (why 78? is this some sort of statistical formula? I don't know statistics or finance...). Here is an example - sorry the formatting doesn't work...



[1] *** [2] *** [3] *** [4] *** [5] *** [6]
N/A *** =[A] *** =[2]*[B]/12*[1] *** =[2]+[3] *** =[C]/12 *** =[4]*[5]

Month *** PY assets *** Change in Assets *** Assets as if Month **** Fee Rate *** Fee
1 2,716.00 (33.95) 2,682.05 0.001098333 2.95
2 2,716.00 (67.90) 2,648.10 0.001098333 2.91
3 2,716.00 (101.85) 2,614.15 0.001098333 2.87
4 2,716.00 (135.80) 2,580.20 0.001098333 2.83
5 2,716.00 (169.75) 2,546.25 0.001098333 2.80
6 2,716.00 (203.70) 2,512.30 0.001098333 2.76
7 2,716.00 (237.65) 2,478.35 0.001098333 2.72
8 2,716.00 (271.60) 2,444.40 0.001098333 2.68
9 2,716.00 (305.55) 2,410.45 0.001098333 2.65
10 2,716.00 (339.50) 2,376.50 0.001098333 2.61
11 2,716.00 (373.45) 2,342.55 0.001098333 2.57
12 2,716.00 (407.40) 2,308.60 0.001098333 2.54
TOTAL 32,592.00 (2,648.10) 29,943.90 0.001098333 32.89

Thank you very much everyone!
I spend two days trying to figure it out...