You cannot try checking sweep and all that stuff on this system if you are running factory Magnum rocker arms. There is only two measurements on these, right and wrong.
The procedure RustyRatRod outlined is 100% correct way to do it on this one. The rocker arm position on a factory system is fixed, so all you are doing is measuring the length needed to take up all slack WITHOUT pushing the lifter plunger down at all, and then adding the desired preload to that measurement. Done.
Try measuring again with Rusty's method and your adjustable checking pushrod and report back with findings.