At first, I was thinking that you are right about keeping the border at 2.5 inches - or something in 2.5" increments. But the more I think about it, isn't the fabric for the sawtooth folded TWICE? If that's the case, wouldn't it throw your measurment off?
Maybe you should make a sample of the sawtooth and measure it at the seamline to be sure. It's been awhile since I did one. I'm sure someone out here has done one more recently than I have and can give you a more accurate response.
DUH, don't mind me...I was thinking Prairie Points. LOL In that case, I would think 2.5 increments would do!