Ti, the focus thing is probably where all the looking through cameras thing made it easier for me.
Don't forget, when we look in any vehicle or bike mirror, our eye is focusing at very far away, well the distance away the thing that we are looking at is actually at.
In camera lens talk, you'd be focusing the lens at close to the farthest away setting, infinity let's say. ( lenses have the feet or meters marked on the lens, and the farthest away mark is "infinity"---- but all this is to say that when riding, our eyes are generally focused on things far away, 50ft, 100, 500 ft or whatever.
When I glance in my helmet mirror, you just "keep" your eye focus at that far off "setting", and you then see the image in the mirror in focus, so you really aren't shifting your eyes focus that much.
Again, I figure because I looked through viewfinders so much, I do this without thinking.