An experiment and how-to mark your stance position...
To experiment with this, a player needs some constant, some standard, to vary from. Here's what I use on my home table...
I put blue painters tape from the center of one side pocket to the center of the other. This makes a blue line across the center of the table. That's my practice shot line. I set up the cueball on the center spot and shoot an object ball that is sitting on the blue line into the side pocket. This creates the same set-up every time so I can practice varying from that standard, that constant, so to speak.
I then get into my stance and look down at my feet. I take two more short pieces (2") of the blue tape and put one behind each heel on the floor, perpendicular to my foot. That creates the standard set-up position. The tape stays on the floor without damaging it and can be repositioned when it is needed.
Now, with my constant, or standard, position being able to be repeated very accurately, I can begin to adjust and measure the results of a changing stance/set-up/foot-position/bridge, etc. If I find, after hundreds of shots practiced this way, that my stance set-up is off a little, I move the heel tape postion(s) just a little bit, to my new standard set-up position. I then practice and test this new position just like I did the last one to be sure it's what I want and need.
After about 3 years of doing this, I have my set-up position locked down tight. I place a cb on the center spot and do my set-up without first looking at the tape marks. I then look down at the tape marks behind my heels to see how close to my standard I set up myself. If I'm way off, I re-do the set-up routine and examine why I missed that mark in the first place. After a while, the accuracy of my set-up becomes much more precise and consistent.
I don't know if that will work for you like it did for me, but what if it does?
One other thing for those with bad backs.....I have to place my left foot off line a little to keep my back from being overworked and stressed, as would happen if my left foot was parallel to the shot line. My left foot now points just to the right of the cb (I'm right handed). fwiw
Jeff Livingston