I'm trying to make a paycheck/commission calculator.

We get paid commission like the following:
25% of GP up to 1000
50% of GP from 1-2k
100% of GP over 2K

I have the following formula in excel to show commission based on GP, but I need a reverse formula to show GP needed to achieve the commission target.

=IF((E9)<=1000,0.25*E9,IF(AND((E9)>1000,(E9)<=2000),(250+(0.5*(E9-1000))),IF(AND((E9)>2000,(E9)>2000),(750+(E9-2000)),750+(E9-2000))))

Any help would be awesome,

Thanks!