I am kind of amazed that I have never given this any real thought in like 28 years of shooting shoulder mount rigs plus more recent small DSLR style cameras with all the endless possibilities of how you can stabilize those.

But if we are talking a substantial weight ENG shoulder mount camera on your right side of course, for long free standing hand held shots, is there a "best" foot positioning that those in the know tend to use? Like should one foot be (how far) in front of the other, and how far apart approximately? Or side by side? Either of them angled (if straight ahead was 12:00 on the dial)?

Thanks for any thoughts. Again I am surprised I have never paid much attention to this and have always been pretty solid with my shooting hand held with full size shoulder rigs. Maybe I should have paid more attention and have been cheating myself ergonomically all these years?

Not that all shoulder mount equipment winds up being 20lbs. But currently when my PMW 320 is geared up with a camera light, wireless equipment, Anton Bauer battery and a Nanoflash recorder for backup on top of the battery, I am pushing that weight.

Never thought about my feet in my time shooting heavy camera rigs. In the old days with 30 pound rigs I think my feet were pretty much the same as shooting a 6 pound rig on a Multirig style mount.
I would put my feet where comfortable usually side by side about shoulder width apart and then when I got tired I would slide my right foot forward about a foot until I got tired then slide it back to side by side.
This only happened though if I got caught "short" say something impromptu like an emergent situation (like a fire or car crash) or a news conference that I would happen upon. As a stringer back in the day it was hard to always be able to use a tripod.
For weddings and other social events if I was off the tripod then it was only for a short time and holding the rig relatively steady for 3 to 4 minutes usually wasn't a big deal.
Just put your feet where your comfortable and steady and if you need to change your position slide your foot don't lift it.

+ for what Don said. In addition, if you're going to pan, point your feet to the spot you're going to END at, then twist your body to the start of the shot, then untwist your upper body for the pan.