You can do this without needing VBA.
Enter your %margin figures in a range of cells. I normally use white text for these and protect them against being over-written.
On an Excel Sheet, View/Toolbars/Forms, Select Combo box and place it where you want on the sheet.
Right click the box, Format Control.
In "Input range" enter the range where your %s are.
In cell link enter the cell address where you want "the answer" - I normally use the cell under the box, so it is hidden.
Note that "the answer" is actually the index number in the list, not the % (in this case). If say your list of %s is 5, 10, 15 then the % would be given by "the answer" * 5%. Alternatively you could use an offset - say your % list starts in A1, A2 etc and your "answer" is in C3 then
=OFFSET(A1, answer-1,0) will give thye % value.