In 50+ years of being involved in straight axle bending, I can't remember ever jacking one in the center to change camber. The anchor point is always just inboard or outboard of the spring, and the axle bending is done between the spring and the spindle, up or down. At least that's the way the Hunter Engineering advanced level alignment school in St. Louis taught me to do it. "Your mileage may vary".
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!