To start with, 13,600 is pretty good ! Id not complain too much about that if everything runs smooth.

A quick and easy way to evaluate pipe lenghth for an existing install.... leave the pipe alone.

install a smaller prop (try an 11x6). See if the rpm jumps a noticable amount. If it does, pipe may be set a bit short, or about right. Leave it there. If it does not gain in rpm, the pipe is working like a governor, and might be a little long.

Install a slightly larger prop (try an 11x9 or 12x6). See how the engine likes it. If it runs well, does not drop a lot of rpm, and transition is unaffected (in otherwords, it runs fine with more prop) the pipe may be a little bit long for the 11x7.5 and you can probably take another 1/4" off of the header or continue to use just a bit more prop. If it acts up, starts to get hot, refuses to transition or surges ----- its too much prop...... shut the engine down and cool it off. In this case, your pipe setting is ideal for your 11x7.5

Bob