How to reverse a formula?

I have a very useful pricing spreadsheet which uses some fairly complex formulas to calculate prices across 3 sales channels at a set profit margin, based on unit cost.

I've attached the file. On tab1 cell E2, you enter the desired profit margin. On tab2 column B:B, the unit costs are entered. Then by dragging down the next 3 columns, you are given the different prices per sales channel.

What I need (in addition to this) is to be able to calculate the profit margin by entering in selling prices and unit costs (rather than calculate the selling price by entering profit margin and unit cost). Can anyone think of a way to alter this spreadsheet to achieve this? Or would I need to start from scratch?

Would greatly appreciate any help.

Thanks.