Excel Formula question
 

I apologize in advance for not being able to frame and explain this properly without having to include a sample file.
Also, I'm sorry if this question is better suited for another group. But I see similar questions and answers here so hope someone who is good at math can assist me.

The worksheet is here, and the problem is described on the sheet.
http://kingfield.net/excel/formula-question.xlsx

In the simplest of terms, I am trying to find a formula which will allow us to adjust our profit margin either up or down. Lets say I want to increase our margin by 3%.
Please the worksheet on my website. It's so much better than trying to explain it.

Thank you, Paul


Cell B4 is calculated based on the % value entered into cell C2. I am looking for a formula to enter into Cell C1 which will return the percentage value that I would need to enter into cell C2 in order to achieve an increase or decrease of cell B4 by the amount shown in cell A1.