It really does depend on your height, strength, and body type. A taller person may hunch over the machine, while a smaller person may have to take a stance that braces themselves against the floor so that they can nudge effectively. I would be reticent to say that there's one correct approach for every person on every table in every situation - the same force behind a nudge in a braced stance versus one in a relaxed stance could, hypothetically, differ in strength enough to be the difference between saving the ball and tilting out. I've heard of people use multiple fingers to gauge how hard they're hitting the flipper button or for timing their flips; everyone has their own systems and idiosyncrasies.

Honestly, there are so many variables that even starting to think about it makes you feel like you're overthinking it, but analyzing your stance and the stances of others is a valid thing to consider when thinking about play style and breaking down how to play the game.