Would be nice to think that there is some kind of formula for this.

Unfortunitly, this is one of those things that varies wildly from tyer to tyer.

Make sure first, though, that you have a good chair. That will solve the vast majority of tying fatigue issues.

You want to sit comfortably without bending over, and be able to easily reach the vise without overstretching or looking like you are holding a shotgun.

By far most folks tie with the vise too high, and after a poor seat that is the most common cause of tying disconfort.

Finding the correct vise height after that will just take some tial and error.

Good Luck!

Buddy