I got used to the finger test when i started using grills at different places. Not a problem if everyone’s using their own, but it is useful when you find yourself cooking at a picnic area or renting a trailer grill or, well, you get the idea.

Hold your hand out, palm up. See that big bulge of muscle at the base of the thumb? You’re going to press it with the forefinger of your other hand.

Raw should feel about the same as when you press with the hand relaxed.
Rare is lightly touching thumb to forefinger (the OK sign).
Medium rare is middle finger to thumb.
Medium is ring finger to thumb.
Well done is thumb to pinkie.

Notice that there is no direct comparison for medium well. You’re going to have to guesstimate if that’s your aim.

For what it’s worth I actually prefer the finger test because the meat thermometer is another hole for juices to escape. That’s the reason for the word nominally in the opening sentence.