Automate Goal Seek Function

I'm looking for help in automating a goal seek formula to work out a margin. The aim is to get a target margin of 20% for all places. The formulas are all there.

Goal Seek
Set Cell AA3
To Target 20%
By changing cell G3

Similarly for row 8 & row 13. File attached.

The issue arises when I'm constantly checking for new prices. I want to be able to insert prices in sheet 2 and generate final sale price for all 3 markets at 20%. Do I have to trial and error or is there another method for doing this. Perhaps adding the 3 formulas on each line of a different sheet.

Variables are 'Item Cost' & 'Postage Cost' (which is based on weight)

I found a similar query but not sure it would work the same:
http://www.excelforum.com/excel-prog...goal-seek.html

Or is there a simpler way of working this out without macro?