You could try a formula like this:

=(C17-(C17/B17*495))/VLOOKUP(H1, I1:J5, 2)

The VLOOKUP requires some explaining.

H1 is the location of the dropdown box.

I1:J5 is a lookup table you need to create. It could be in hidden cells, or it could be on another sheet. In this case, I said that column I is the value of H1 that you're looking up. Column J has the number you're dividing by (such as 1.2).

If you can re-create that, then this may do what you need.