I read the same article many years ago and have indeed used Ken Waters' data. I was actually using Accurate Magnum Rifle Powder, which was termed as a 3100 data powder. My dad bought an 8 pound jug because it was on sale cheap back when I was a kid.
Here is my recollection:
Basically figure out how much powder you need to fill all the space in the case with your bullet seated to the right depth. The powder is so slow that you can't over charge a 45-70.
I loaded mostly 405 grain and 500 bullets. They left lots of partially burned powder grains in the bore and the cases were very sooty because they didn't expand to seal the chamber walls. Pressure was certainly low and I recall chronographing some back over 20 years ago that I think were around 1100 fps.
Tried using some 300 grain bullets and they had very poor combustion. Very weak. There wasn't enough bullet mass to hold back the powder to get it burning.
You need to occasionally blow out the fouling or those partially burned grains will fall back into the action and start to bind things up. I was shooting a rolling block which was easy to clean. Working the loads in a lever gun would probably dump those grains into the action and make things crunchy if not bound up.
If you've got a bunch of 3100 and want to burn it up with heavy bullet plinking loads, this will work. There are lots of other cleaner burning options though.
Ken Waters' "Pet Loads" book and all the supplements to it is a classic. One of my mentors gave me a copy of that book many years ago. I used to pour over that book and study all those articles- even for cartridges I'd never seen. Ken Waters was an old time gun writer and I wondered what happened to him after I stopped seeing his articles. He lived until fairly recently.
https://hoytfuneralhome.com/tribute/.../obituary.html