I leave both small adjustments closed, then adjust the large slide out only, until I am two or three grains under target. I try to be close enough that only a couple turns, or less, of the largest of the two little guys is required to bring up the right amount.
This is for rifle, where I seldom, or never, use the smallest one.
For small amounts of powder, like 8 or ten, I just use the two little fellers.
On rifle I am usually within one tenth, or a hair more, on powder that cooperates.
It is imperative to always operate the handle the same. I try to click the stop on opening and pull it back with the same speed and pressure every time, as near as possible.
I was having the same issue with mine. I put in a baffle and it solved it.
I have been using my 55 for over 40 years. First, I close all slides up. I never use the two small slides when loading for a rifle. I use a Rx torsion balance to get my desired charge weighed, dump it into the 55, then gradually pull out all slides together so that the weighed charge appears to fill the "measure", tighten down the two set screws, dump in more powder, slightly over 1/2 full, drop in funnel with about a 1/2" bottom opening, add powder to the funnel.
On the little swinging hammer, I have elastic bands so that when I have dumped, I flick the hammer 3 times, thus ensuring no grains have hung up inside the measure and settling the powder in the reservoir, the funnel IMHO doing a better job than baffles.
After dropping about 5 charges, back into the powder can, I drop a charge and weigh it on my balance. Usually my weight is off by several grains and I then loosen the two screws and try and move the main slide very slightly in or out, without touching the two fine adgustmnet scews for the two small slides and then tighten the two set screws. This is a hit or miss job and with granular powder, & will rarely get it within 1/10 of a grain. Ball powders, of which I have only had expereience with Ball C(2) and H450 are another story. I can usually get the adjustment right on.
Although H4198 uses a small slide for a final adjustment,I try not to because doing adds to the crevice, corners inside which the larger the powder grain, the more deviation.
Another thing, if a long grain powder is being used, do not pull the lever down gently. Do it in one pull, ignoring the slight hesitation when you know that grains are being cut. (Another reason I love ball powders.)
I drop into a loading funnel with the case mouth inserted into the lower end of the funnel., being sure that all pwder has entered the case.
Obviously, to take the time to set the 55 exactly to throw the required weight takes time and IMHO, not worth it for only 20 or 30 rounds. With a
Rx balance, with agate fulcrums or even torsion, I never wait for the pointer to center and stop! I watch to see how far on each side of center, the indicating pointer goes, watching as the oscillations gradually decrease and decide when I have the correct weight, lifting and locking the mechanism. This past week, I weighed 35 charges of 4759, and know that they were all within one powder particle of each other, one being sufficient to change the weight perceptively. This precision was not really required but old habits die slowly.
If I were working up a load going for accuracy and velocity, for one rifle, I would weight every charge as decribed above. Once I found the correct charge, I would then take the time to adjust the Lyman 55 as decribed above, and load sufficient ammo to make the whole process worth while.
And I have at times, got the 55 to within a grain, then dumped into the balance pan and adjusted with a small white McD coffee spoon.
The object in reloading, as in so many aspects of shooting, is to standardize on a procedure that works and then not to deviate from it.
My reloading over the past 15 years has been decreasing and never approached the volume and experience of so many who post here.
Never let it be said that I was sparse with the words.
