I'd be looking several other places before a scale in the case of reduced accuracy, 2920 fps on a previously 3000+ fps can simply be change in the lot of powder used, and manuals are notorious for optimism compared to hunting rifle realities. I'm pretty sure your scale is probably just fine. As for lubes, a dry graphite powder won't contaminate it any more than the graphite powder that's already part of the coating on the powder kernels, IIRC.
