Hold your fist in front of your face with your elbow bent at a right angle; use your knuckles to select the lens you want for the scene behind your hand:
4knuckles:50mm, 2:100, 1:200.
The trick is figuring out where to hold your fist so the above rule holds. Try it a few times.
Take a photo of a scene with a 50mm lens (or look through the camera's viewfinder with a 50mm lens in place.)
Put the camera down & hold your fist in front of your face such that the 4 knuckles just fill that scene height.
Remember where to hold your fist next time you want to select a lens.
Dave in Iowa