size of prime

That sounds about right for a Nelson. Fox , MK III, IV, and VI seem to take a little more, amd one or two flips backward to suck the prime down into the case a mite as did the old G21-35 Tigres.
It is a fine balance and usually a slight bit different between engines.
A good way to srart without getting your fingers whacked with a "Fingermatic".
Bigiron

That might indeed be right for a Nelson. I haven't actually had one, and I can't remember off hand how much of a prime people gave it when I was involved with the pitting. For a Fox, the usual advice was to fill up the exhaust stack with fuel with the piston closed.

I can't remember having backward running engines except at one Frozen Finger where the temperature was around 20 degrees (and the wind was pretty fierce). Bud Bodzioch said that for some reason Foxes liked to run backwards when it got that cold (and, in this case, it wasn't a brief backwards run - they'd keep going if you didn't pinch off the fuel). We used lighter fluid to prime the engines. I stuck those hand warmer packs inside my starter battery cases.

One thing I'd heard some people do was to use an electric starter to distribute the prime through the engine. Do this before the start signal, and it'd start reliably upon a backflip. Never actually saw it being done, though. I imagine it'd be bordering on the controversial, like when Ed Brzys used a bladder-powered flood-off on a SLOW combat plane.

Well, you want to talk real little piggy's, my SuperTigre 60's all seem to need between 6 and 7 chokes, fast flipping chokes at that, not that slow stuff, then just as many fast flips afterwards to atomize the fuel. This is a border line flood probably. Hook up the battery then spin of the spinner with my fingertips back against the compression and vroom!

I've never had an OS/FP/Fox35 etc that would take more than 3 chokes to prime, if I did, it would be flood city. If it takes more, you must be running the engine at a lot leaner needle than I do, or your tank is miles away.

My four strokes will not stand more than 2 chokes with out wanting to hydro lock. Since I went to the electric starter, I don't even choke, I just let the fuel draw up from rotation of the starter.

I guess that really is a lot of the equation, how open is your needle to start with from it's previous flight. Every installation would seem to need it's own starting method.