I didn't see it posted here, so here goes.
You only base it off of 13.5 inches if your racquet is 27 inches long. 13.5 is half of 27, if you didn't catch the correlation.
If your racquet is 27.5 inches, as some are, then it would be 13.75 inches.
I love the balance board though. I'm gonna make one for myself now that I've lost the use of my RDC.