Starting with the 310 IBO rating for the XTec...

- loss of 30 fps for that 3 inch shorter draw length you are shooting.

- loss of approx. 8 fps for the heavier arrow weight

- loss of anywhere from 3-10 fps for the string accessories you may or may not have on the string.

310-30-8- (3-10) = anywhere from 262-269 fps.

Your chrono sounds to be right on with my estimates.