There can be no set formula. I've seen courses where the "middle" tees were over 6700 yards, and ones where they were under 6200. You have to take length and general difficulty into consideration.

Often a course that you don't know will be more difficult than one you are familiar with, even if the slope and rating make it sound fairly easy. I usually approach a new course fairly conservatively. I think I get more fun out of a golf trip that way, and then if I play it a second time and feel that I want more of a challenge, I may move back one tee.

I don't play golf to torture myself, especially on an all too rare golf vacation. I'd rather have fun, and sometimes that means playing either the middle or senior tees and ignoring my ego.