Weakhand support
jcl89, When we run our Garand clinics we teach that the weak hand should not "grip" the forearm of the rifle but rather support the rifle. I think of the weak hand as I would a front shooting rest off a bench, ie. support the rifle don't hold it. I believe this applies to all rifles in position shooting. This technique prevents contact with the op-rod and prevents muscle input being applied to the fore part of the rifle when shooting if you change your grip pressure.
Generally, most shooters move the left hand as far forward as possible when using the sling in prone or sitting.
In the off-hand position there are many different ways to support the rifle; left hand cupped rearward around the magazine, left hand reversed with the thumb on the right in front of the magazine, thumb on the left in front of the magazine etc.
You may want to look at some of the USAMU videos on the CMP website.
I hope I have helped some. Good shooting. "T"