What is a good weight to start with? Or, a formula like "If you can do 100 one-arm KB swings in 5 minutes with X weight, a starting mace weight is Y"?
Hard to base it off of a KB swing standard, as those are much more leg/hip dominant. I started at 15lbs and there was a learning curve. I think if you've not used something similar like heavy clubs going a step lighter if you're stuck between 2 weight choices is the best way.
I'll speak to practice of the 360, because really that's where you should focus. It's very rotator cuff intensive (well the whole shoulder girdle really). If you need a KB standard as reference it would probably best be based off looking at: 1) Snatch; *and* 2) a press or TGU.
When I took up my 15lb mace practice I absolutely owned the 20kg for volume on Snatches, and could press/TGU/Bent Press the 28kg for volume on any given day (this was the heaviest bell I had at the time).
I worked the 15lb mace with 10-to-2's first, doing sets of about 8-10 reps with both right hand dominant and left hand dominant grips, and built volume. I moved on to 360's when I felt in control of the weight and lever length.
The mace plays well for high rep work (by this I mean 25 on up into the hundreds of reps), and I've found 2 technique dependent areas you can alter your swing to emphasize. These would be: 1) shoulder opening/mobility, 2) full body integration with strong oblique and QL emphasis. Basically it has to do with how much you shift your weight from foot to foot as you swing, and how much you lock down your core.
I found a 35lb mace for a steal on a going out of business sale, and bought it. It was a real bear making the jump from 15 straight to 35lbs. Once I could get 10 reps with both hand positions I was off and running and built serious volume with the 35 though.
The only issues I've experienced are: QL inflammation and the fibrous/fascia area of the upper arm below the triceps can get sore. The QL issue was from too much volume to quickly (so my error lol). The soreness below the triceps seems to pop up when you're new to the mace, coming back from a layoff, or when you make a weight jump. I've remedied it by lacrosse ball rolling and self message. If it presents, I work the tissues and I'm right as rain. Point being, build slowly and respect the thing (it's a medieval weapon after all).
Overall I think it's a great tool and will probably continue to swing the mace for many years to come. You can really get into a rhythmic zone where you experience that nice time distortion of being "in the groove" if that makes any sense. I like what it's done for my shoulder health and torso.