That's actually NOT how you go about figuring how to get to "13%", but allright.. check your numbers from line to line, too. Anyways, with your weight at 270 @ 21% Fat mass, which means 213.3 lbs of Lean Body Mass. If you were to drop off 21.6 lbs, that would of course put you at 248.4 lbs Total weight @ 13%, which would result in 216.11 lbs of Lean Body Mass. Which is actually better, since you will have gained a whole 2.81 of muscle.
There is a formula for figuring it out, the easy way. I just went with your numbers, to show you what you're trying to do, with what I know there.