running total subtract % of previous months

I need a formula that keeps a running total of the following.

Add 91.6667% of current month, subtract 8.3333 from the previous 11 months.

So, January should be 91.667% of 100

February should be the amount from January less 8.3333% PLUS 91.6667% of 90 (february numbers)
Etc Etc

Spreadsheet attached.