Double 16 is most forgiving because double 8 is above it. If they hit single 16 they can go to double 8 above it and if they hit double 8 above it, they go to double 8 again (requiring minimal or no correction)
There are some players who like double tops (20) but for the most part double 16 is the conventionally wise way to check out.
Darts dip a lot in flight - they do not move side to side as much. As a dart player, I guesstimate that after only a few hours practice most people will be off 1.5cm or so in the vertical but only 0.75cm in the horizontal
Professional dart players are always aiming at 20. If they block the triple-twenty with their first or second dart, they then aim at triple-19.with some players preferring triple-18 since height-wise it is in similar range
But does the variation in the two different axis result from "how the dart travels", i.e. the physics ? To me it seems to be a matter of relative consistency of the angles with which one throws the dart. I remember reading an article that analyzed trajectories of basket ball free throws. They found for a given player their velocity of projection remained surprisingly consistent. Maybe that is true for throwing darts as well.
- relevant statistician joke
I really like the way you are using your technical skills for computation but I think you are using it on the wrong problem.
In reality, even if your dart throws are Gaussians they need not be independent of each other.
I'm coming from math background and was observing math people(physicists as a proxy) to the approach of making unreasonable assumptions if they will simplify the problem and make it solvable.
It reminds me of the joke:
A person is walking around a street light. Another asks him
"What are you doing there?"
"Looking for my keys."
"Did you lose them there?"
"No, but there is more light here."
Edit: formatting
There's only one more thing this needs to be perfect. I'd love to be able to calibrate it to me, by aiming for the bullseye 10 times and telling it what I hit. That way, it could assess my expertise, and suggest what I should personally be aiming for.
The second is that in my experience, dart throws aren't normally distributed: they're skewed towards the bottom, because I don't throw very hard. I wonder if that effect goes away with practise.