+/- .1 or even .2 grains is NOT going to change velocities by 100 FPS!
Think that through...We push powders up 5-6 grains to get that much velocity increase quite often!
Thus, I doubt that it is the scale, but borrowing a quality lab scale to give you some peace of mind about it isn't a bad idea.
I found that my velocity variations DROPPED significantly when I started using a Chargemaster vs. a beam scale. Beam scales can be very accurate, IF everything is perfect and consistent, but most guys using a beam scale will be NO more accurate than a Chargemaster. Beam scales are very dependent on you having a view of the scale that is consistent charge to charge, and most guys aren't good at that! (To achieve best accuracy, you should have a perpindicular view of the beam, and most reloaders don't do that, because it requires getting the scale to eye level, or bending down to it. Try bending to the same exact spot 10 times, much less the 100 or more that most reloaders use each time!)
I have fiddled with my Chargemaster, and it is faster than it was from the factory, and I haven't seen an great degradation in accuracy.
Large extruded powders don't meter quite as well as fine powders like Xterminator or TAC, but the soda pop straw trick helps with that issue, too.
I think there is something more significant than simply the powder charge causing that kind of variation!
BTW, my 7mm WSM
varies less than 20 FPS with a WHOLE GRAIN of H1000 at 2930 and 2950 FPS between 64 and 65 grains. 66 grains pushes it only to 2960-2965 FPS.
Look elsewhere first, once you have convinced yourself that the scale isn't grossly off!