I think the touch hole needs to be about 1/8” or so in front of the breech plug to avoid being affected by fouling during competition. When the touch hole is too close to the breechplug, or even behind the plug face with a groove filed, there is an increased chance of uneven ignition caused by fouling.

My most accurate flintlock has the center of the touch hole 5/32” in front of the plug.

I've seen a test somewhere that tested ignition location, but I don't remember what the conclusions were, mainly because the location was at extreme/ideal positions, e.g. inline, middle of powder column, top of column, irrc, and thus almost completely irrelevant to anything I'd normally do...

I agree about the hole being somewhat ahead of the breech face reducing fouling problems both when wiping and not wiping.  Appearance-wise, however, it is better to put the touchhole as far back as possible, especially with the longer breech plugs used today.

If I had to bet on a best accuracy option, it would be inline, rear center.  That can be done with a percussion gun fairly easily, but you would have to use something like a true patent breech (Nock?) with a tiny powder chamber under the main charge on a flintlock.  That is just a guess, though...

I’ve always done a nock style breech on flint and percussion. The advantages are, center ignition, and if you dry ball, you can get powder under the ball. Of course I’ve never dry balled. Lol