Hey Joe,
The Lyman 55 was my first powder measure, and I would guess that I have had it nearly 50 years now. My second powder measure was an RCBS that I got not too many years after my Lyman 55. My third measure is the Hornady case activated measure that came with my Hornady L&L AP progressive press I bought in 1997.
I like all three of them, and I still use all three of them for different applications. The Hornady measure is used strictly on the progressive press, and the other two are used for doing work done on single stage presses. The Hornady and the RCBS kind of work in a similar fashion with their "screw plug" thing to adjust for powder desired.
The Lyman 55 is really a different breed of measure in how it adjusts for powder charge size. Like the others, there is a rotating drum, but the adjusting mechanism is quite different on the Lyman 55. It actually has three adjustable sliding parts incorporated in its rotating drum.
I know some folks say to use the smallest slide for the smallest loads, the medium slide for medium loads, and the largest for the largest of loads. I think that idea may be somewhat misleading. I would never advise opening the smallest slide all the way before beginning to open the second slide to increase charge weights.
My recommendation is to open the slides together with the idea of ending up with a "charge hole" that is nearly square in shape rather than having a "charge hole" that is real wide and very shallow. I hope my description makes sense as I have written it. I would also recommend that no slide should cover any open part of the slide under it.
In other words, if your charge was sufficiently large to warrant opening all three slides, the middle slide should not be cover any part of the larger opening of the bigger slide, and the small top slide should not cover any part of the midlle slide. And hopefully, the resulting charge hole should be pretty close to being a squared off shape rather than an oblong shape. This makes for the smallest opening required for a given charge and less edge for powder to be cut as the drum is turned.
I would think this may sound a bit confusing to folks who never saw the inside of a Lyman 55, but I hope it makes some sense to those who actually have seen them. I think the 55 is a pretty good measure, but if I am using extruded powder, I will generally use the RCBS because it has less of a cutting edge than the 55 when dropping large charges.
Best wishes,
Dave Wile