I have an .078 on my offhand flinter. Great ignition. It was stock, maybe .055, and I got pretty good ignition with that, but it was fussier. An occasional hangfire, more that I liked. But with the .078, it goes off anytime there is fire in the pan. I do spray anyone to the right of me with grains of 2f more than it did with the smaller hole.
A question here: Can a vent fire out UNBURNED grains of powder? It is my belief that it can. Because the guys 20 ft down line have picked them off their jackets or out of their hair. I now put up a piece of cardboard on the shed post to catch my drift.
So if a vent passes powder, unburnt powder, that means there is less of a charge to fire the ball. How much passes thru the vent is proportional to the size of the vent and the pressure in the breech. But if the amount of gasses and powder passing thru the vent is variable, there goes accuracy. I assume it is variable, and I also assume the bigger the vent the more pressure fluctuation you'll get.
So I am of the persuasion to keep the vent as small as possible. Bring the size up until you get reliability, then stop.
Caution: flint shooting can be addicting, and there is no known cure.