No power check-die here. I use a 550 and my experience and procedure is pretty much the same as yours. I’ve had two experiences with incorrect power charges. The first was under charges. Like you I look in just about every case and weigh a charge every now and then just for grinds and giggles. I noticed the power level looked low in a case so I weight it. It was! Next few were on the money then another low. I took the powder thrower apart and found a small piece of paper/cardboard, (have no idea where it came from) in the opening above the powder bar. The only over charge, more than + .1 gr. happened when I short stroked the handle then completed the stroke. When the powder spilled over the top of the case I knew I had done a no no.