Margin Calculator Help Please

Hello All, Any help greatly appreciated here. I currently have an Excel Spreadsheet whereby I input a suggested sell price for a product and it provides me the Gross Margin. This spreadsheet takes into account certain variables, such as material cost, postal cost and selling fees (which are a percentage of the ultimate sale price). Is there a way whereby I can input the desired Gross Margin and the spreadsheet will calculate the sell price including the variable selling fees which are a function of the ultimate Sell price?

Any assistance greatly appreciated...

Cheers, Tony