Dave,
Is that one of Frank Zika’s sights?
I got one with a rifle I bought a couple years back. It was more math than I wanted to deal with. I sold it to someone on this forum. Maybe he’s figured it out and can help.
JKR

My experience  with Vernier sights, Mike Stevens & MVA you can't count on the gradations to relate exactly to MOA. Not just the gradations, rear to front sight distances can vary too. Another complication few rifle ranges are measured precisely.

I have two CPA's and 4 barrels all with the same sight distances and sight graduations. One graduation is a slight bit more than 1.2 MOA . Split  between two gradations the sights measure to 06 Moa . All this from calculation, proved on targets with settings used in matches recorded in a notebook. 

Sounds complicated but it's really not, once recorded you can dial in precise point of impact fast. Silhouette sports varying gun club ranges 4 banks of animals essential 

Boats

Traditional Vernier tang sight staffs were graduated in 1 inch increments, divided into 20 parts. The vertical slider was 1/20th inch divided into 6 parts. The illustration below (from my book Loading and Shooting Traditional American Schuetzen Rifles) shows how to increase elevation in one minute of angle (MOA) increments or 10 MOA. This assumes a barrel length of 30". Your results WILL no doubt vary but this is how to read a Vernier scale.

Dave,
This staff is calibrated for a 28-29" sight radius and with that it does give you 1/4 MOA adjustments.  That is providing you accept that a minute is one inch at 100, two at 200, etc.  It does look like a Zika sight and comparing it to an MVA that has 6 graduations (0 plus 5 more) on the traveling block (each line move is .010"), this one has 21, (0 plus 20 more)  each line move is slightly less than .002", or almost 1/4 MOA - as .008" is a MOA.  confused?  As Boats shared, from Hatcher's graph it's about a 29" sight radius and there are rounding errors.
   
Most of my builds get set up so the sight radius is 34.378" or so a minute (1.047") works out to be .010" on my staff.  Lately, on another subject similar but different yet, I've been graduating my scope turrets and mount spacing so I get half MOA corrections.  It's anal and I can't really shoot that close beyond 100 in most conditions but the mindset is there.

Back to the irons, with the naked eye most folks can barely resolve a minute, a lot of our gongs out to 1200 get a 2 minute dot painted on them, there's a reason for that.  Can you clearly see a one inch dot at a 100yds? I rarely can anymore.  Now if you could stay juiced on adrenaline it'd be clearer and stay sharp, some activities create that, most target shooting not so much - competitve competition between a couple shooters maybe... Hunting... or battle...
GT

My kind of vernier scale. 

Not me. I'm like GT, can't see or shoot good enough to use a fine scale. Here's yet another scale on a Frank Zika sight.

"If I had to do it in Mil's would be totally lost"

It's not that bad, just more numbers...  It is much better if both the shooter and spotter are talking the same language though.   



I get a chuckle out of folks that use nothing but mils but ask "how many minutes high was I, or I shot a 1/2 MOA group"  correct their answer in mils instead, causes pause.  If you know they use a scope graduated in mils, then they use a vernier tang sight - give them corrections in mils, it throws them off.