Yakfisher,
Log onto the Goldtip website. They have their arrow selection charts there as well as a "Build your own arrow" function. You can plug in your specs and it'll give you the Forward Of Center percentage. Keep in mind that it's not going to be precise, but you'll get a good idea of what your FOC % will be.
Also, 26 1/2" GT 5575'sshot with a 64# bow will be spined good for a 100 grain broadhead.