Previously I used 3 simple formulas (if statements) to calculate a
charge, so if E9 =£3000 then the formula for the charge was

=IF(E9<=1000;E9*1.1;"£100") then
=IF(AND(E91000;E9 <=2000);(E9-1000)*0.9;"£200")
=IF(E92000;(E9-2000)*0.5;)
Total Cost ... of above 3 items plus E9 (£3000)

Now people are asking to spend a total of £3000 including the charge,
so now Total Cost is to be £3000 and then I have to work back to find
E9.

How is the best way to do this please.

Bob