Margin/Markup Option

I'd appreciate a way to calculate pricing on Quotes and Invoices with 'margin' and 'markup' options. Let's say I want to hit a 30% margin, I can't just input 30 into the markup field, I have to go make a calculation on the side to get to the correct markup number - in this case it would be 43. It's easy to see how off my goals I am if I don't take that extra calculation step.
As I am paying close attention to my income these days, I want to make sure I am not making a simple error that costs money to the bottom line.