we've/I've covered this before.     you are exactly right k-mech, but depends on what your end goal is if all a person wants to know is how much corn is in a bin, or documentation for crop insurance, then the 30 minute calibration is good enough.

spent the better part of two hours calibrating a force plate system last year in a heritage case ih combine. had it with in .5%. all afternoon, two days later, in a different hybrid it was consistently 10-12% too low, got back into the same hybrid it was calibrated with, it was back within 1.5%. 

if you are comparing hybrids I agree with wtw, impact plate systems calibrated to the nth degree still aren't able to give info that can be construed as reliable, although in our yieldchecks the oem yield system in the flagship combines comes pretty darned close.