|You could use a lookup field based on the dollar amount and then make the calculations.|
I would fix a cell to hold the monthly exchange rate - you could change the exchange rate and not have to alter a constant in the formula by using the cell reference for the exchange rate.
So, you enter the dollar amount, which looks up the percentage for the next cell, uses the exchange rate and delivers the calculation to the cell you desire.
If I am not understanding your need, please comment further.
I'm still learning everyday.