First and foremost, it is important to measure around the clasp of the chain you will use for your pendant to dictate the circumference required for your bail. Before you start bail construction. If you have a special clasp you want to slide in and out, then:
Bail Circumference
Measure around the clasp with a soft measuring tape the minimum length required to comfortably slide the clasp in and out and then that should give you the length of X-wrap to make before creating the loop.