Excel 2003 formula for profit margins

[Americana]

Good Morning All,
I am trying to make a spreadsheet that has figures profit margins for dollar amounts from $250.00 to $7000.00 with profit margins on those amounts of 18% to 30%.

I have originally set up the sheet $$ amounts in row (A) and %'s across the top.

Example:
Invoice for $850.00
Pay Driver 700.00 of the 850.00
for a profit margin of 18%

I can not figure out the formula - can any one help????
Thanks

[ychartra01]

Retail = Cost / (1-margin)
Cost = Retail x (1-margin)
Margin = (Retail - Cost) / Retail

Margin = (850-700)/850 = 17.6% marign