the old standard for bmx frame measurement is in inches, and measures the center to center measurement of the top tube from where it meets seat tube, to the head tube juncture, just remember to mark the centerlines of the seat tube and head tube to make that easier.
Remember if your height is about 5-7 but the frame is a pro Xl with a 21 inch top tube, you have way more leg room than you need, and with more wheelbase that handicaps you with jumps, turns etc. Greg Hill raced on a GT pro that was smallish in length (18.5") and he was of avge height as a top BMX pro.