What are the tolerated variances in things such as grains of powder when brewing a plinking round? Is a .3 swing acceptable? I would think that you want it as close as possible to your desired weight, but knowing the Unfilow isn't perfect, and we really can't weigh EVERY throw, there will inevitably be some variance.
A 0.3gr swing on a 30gr charge should be acceptable for plinking ammo -- assuming 30.5gr is below the max charge listed.
A 0.3gr variance for a 4-6gr pistol charge is generally not a good thing.
If your powder thrower is being inconsistent there are a few likely causes.
User errorThis would be typically not having a uniform / standard operation of the throwing mechanism. There is a 'knock knock' pattern you should get into when throwing charges. Basically, every force / timing of manipulating the powder thrower handle should be the same. It'll take repetition / practice, but you'll get the hang of it eventually.
With progressive presses, this means raising / lowering the ram in a consistent fashion as well (which can be interesting when cases think differently of being sized heh).
With a hand activated powder thrower, you can try flicking / tapping the drop tube or cam with every charge (or twice, once on drop once on refill). Might help. Might not
.
Static electricityIs powder sticking to the metering cam ? Or the drop tube ? Grab some Bounce or other anti-static dryer sheet and rub it over stuff. If you're indoors, particularly during the winter with the heater running and no humidifier, static build up is very easy to accumulate.
MoistureIf it's hot and muggy in your loading area, powder might wanna try to clump or similar.
Powder choiceSome powders simply don't like to be volumetrically measured. Some of the slower burning rifle powders with long rod shapes can make this a bit worse too. Nothing you can do about it, but you should eventually get a rhythm or setup that should make drop nicely.
Powder measure creepSome powder throwers / measures will creep on you. Verify powder drop every X number of throws (10, 20, 50, 100, whatever you discover is a good number after a while of using your thrower).
I've not done enough work with the cheapy Lee Perfect Powder Measure yet... but it was wanting to be off by up to 0.4gr when working with a rifle ammo. I need to fiddle with it far more to see if I can make it purr better. But due to inconsistency I ended up measuring each charge I dropped out of it, and trickled some powder in if it was necessary.
On a side note, a shouldered rifle case can make a fine substitute for a powder trickler. Just put some powder in it and slowly rotate the case to coax powder out.