To be entirely fair, you didn’t word this the best because it sounds like you’re saying that the ignition percentage is 25%
“It’s about 50% chance per hit, and after 2 hits would be 75%, you got unlucky and got the 25% chance of no fire.”
Don’t forget either there’s a dark orange/reddish fuel tank as well that didn’t go off either.
It still counts, and if we go back to the vulnerability tests, which gave less comprehensive exposure on exactly HOW incendiary works and more along just the strict data. We have hits on the A-35 (P-47 stand-in), B-25, P-59 and P-38.
But if you want direct armor.

This was from 400 yards with M20 API-T with the tank 3 feet behind a 1 inch plate
The report for Aircraft Vulnerability and overall armament effectiveness states that M23 and M20 are “Practically the same”

I would imagine this is due to the velocity of the round helping with armor penetration

It’s actually a little faster compared to in-game 16m/s more.
but they used the P-38 fuel tanks for the chart.
Much of the direct incendiary study was not specifically geared towards just P-38 tanks
But for M23, it’s safe to say it was less than the M20 API-T.
With much of the penetration of the M23 coming from it’s velocity.
They did the same with japanese fuel tanks with dural screens rounds had to pass through multiple of.
Again, these are fast moving rounds going through aluminum. They’re not going to start flipping all over the place like this
This is what M1 incendiary looks like right before passing the last ‘dural screen’

Still straight. Nose broke off.
But if you really want to. You can use the WT wiki’s Demarre calculator to see how the pen of the rounds would look in gaijin’s eyes. Before you say “This doesn’t actually work.” or “It spews out a random number” Put in the stats of the .50 API, API-T, and AP round in-game and it will give you a value.
Since gaijin doesn’t count incendiary filler as part of “explosive mass” I decided to include the weight of the filler in grains converted to kg as part of the explosive mass, as it makes no sense for the M23 to have more pen than the M20 API-T (only by 1 mm)

I want them to have what they realistically had at the time. Even if they had the proper M1 incendiary composition, you’d still be reliant on actually hitting fuel tanks. Or y’know. Just custom ammo belts for everybody.
And these are machine guns meant to catch people on fire. It shows the weaknesses of point convergence when you can’t saturate an area since all your guns are converged at one point.