I've never owned or used Herrel's powder measurers. Of those listed, the Lyman #55 is the most consistent. I have the chargemaster's predecessor. The RCBS digital scale has a listed + or - of .1 grain, this makes the window actually from-.15 to +.15 making the window .3 grains (about the same as Lee Dippers). Depending on the powder the Lyman runs closer to .075 grain +/- making the window .15 grains! Measuring these requires a scale that is better than 1/10 grain +/-.

My best powder drop is a Redding BR 30. I find the consistency to be closer to .05 grain +/-, making the window .1 grain, which is three times smaller than the two systems you are comparing! The first mentioned Herrel's powder drops (depending on the model) are believed to be in the .025 grain +/-, making their window .05 grain +/-! (I don't know a single shooter that can actually use this kind of consistency to their physical advantage! However, knowing your ammo is the best there can be, has a definite psychological advantage!)

Ivan