Riders measure the rein by pulling the rein snug across the fork (front) of the saddle with their fist against the back of the fork and their thumb extended.
Most riders consider a fist and extended thumb an "average" amount of rein. For the horses that buck with their heads way down, they add to the average, and subtract if they do the opposite and buck with their head up more.
Broncs, like we humans, have their habits. But, also like us, they have their days when they break those habits; sometimes to greater extremes than others. Therefore, the rein measurement is often not correct.
I once heard a California bronc rider, Bill Nelson, say "You don't measure the rein to get it right; you measure it so it's not so far off."
Missoula, Montana, saddle-bronc rider Bill Lawrence was once giving advice to a fellow contestant to shorten the rein on a certain bronc. He said, "If you were supposed to push on that rein, they'd give you a stick."
World-champion bronc rider Alvin Nelson from North Dakota went to a rodeo at Red Bluff, California. He'd drawn War Paint, professional rodeo's 1955, '56, and '57 bucking horse of the year. Canadian bronc rider, Les Johnson came up to Alvin and said, "They're all giving him too much rein and that's why he's bucking 'em off."
He asked, "Have you ever seen a picture of him with his head down? Just give him this much, at most," as he held up his fist. "If you give him more, and it don't work, I'll take your rein and beat you with it!" Nelson took his peer's advice, spurred the pinto bronc to the buzzer, and left town with the first-place paycheck.
Bill Hancock, a New Mexico saddle-bronc rider, placed high in a go-round on a bronc named "Rough Going" one winter at the Denver rodeo. He'd received a nice-sized paycheck, since Denver was, and still is, one of pro-rodeo's best paying events of the year.
Bill wasn't known for being conservative with his money, rather, the opposite. About two weeks later at the Fort Worth rodeo, another bronc rider drew Rough Going. He hunted up Hancock, seeking advice on how to best handle the bronc. Bill told him; "Give him this much rein (holding up his fist and thumb). I rode him at Denver and won enough money to last me almost two weeks!"
Another time at the Denver rodeo, Tom Tescher, my dad, noticed that fellow North Dakota bronc rider Joe Chase was looking pretty sickly. He asked Joe what was wrong. Joe said, "I've got Asiatic Flu!"
"Oh," said Tom, "I've never heard of him. How much rein does he take?"