If its positive the first time, you pick a higher percentage as the project is clearly giving you more than the initial % you selected. I usually start @ 5%, and work from there, either going up to 10% if my first answer in positive, or down to 2% if negative.
Even if you get two positives which I sometimes get, it should still be fine when using the formula. Just make sure you are entering the numbers in the correct order in your calculator.