Thoughts:

1. Don't expect a consistent tension all the way around with vellum; you'll go mad.

2. The older the head, the stabler it will be at a high tension. I've had the same head on one of my Stewarts for 45+ years, and have gradually sneaked it up to about 90. It stays put regardless of humidity changes, and always sounds great.

3. I'm taking ever longer to bring a new head up to full tension - weeks not days.

4. I'm not sure what effect the thickness of the skin has. Just had a .009 head on my Tubaphone split spontaneously, after sounding great at 85.

Humidity tension adjuster: just slide it toward the bridge or heel as you desire.

Description for the original photo:
Bill Talley said he got it from someone else but doesn't remember who. I found out the following information after I posted the picture. Here's a trick from Elias Kaufman, foremost authority on S.S. Stewarts: cut a wine-bottle cork to wedge snugly between the dowel stick and the head. Place it a few inches from the heel end of the pot, and experiment to find the best "sweet spot" position. This expedient works well on open-back banjos with dowel sticks, and might work well on those with resonators, although I haven't tried this. Here's probably Bill's source ARCHIVED TOPIC: Wine cork as tone/volume enhancer in open backs
http://www.banjohangout.org/archive/114091

My instinct is that in relation to calf skin, a drum dial becomes a device uniquely designed to split a head.

I’m fitting and playing calf skin every day on a wide range of banjo type. Finding your optimal tone is a fairly rustic but efficient combination of gradually adjusting tension, monitoring by pressing the skin in various places across the head, and most importantly playing and listening.

No natural skin is uniform. Bring each one up to, past and backed-off to where the tension sounds best for that particular skin. Concern about exactly even number of half-turns of the wrench all the way round is much less relevant than keeping an eye on level and your ears on the tone. Before you reach a tension that is likely to split the skin the tone will start to sound constrained. Just because an apparently dry skin doesn't tear there and then is not reassurance. It'll probably split as it dries out fractionally further in 24-48 hours.