Simple, dopey rule of thumb:
  • 50mm - one eye open
  • 35mm both eyesopen
I spent a couple of weeks years ago training myself to know the lens field "naturally" -- that is, to look at something without finder or camera and think "from here that part of my visual field fits an 85mm" (or alternatively, "to get this shot with the lens I have mounted, I'll need to stand over... there").

It wasn't hard, and is a skill that has stuck and served me well. Kind of like musical pitch training, I guess.