hey there anya,
IMHO, as far as in the pocket, you'd be looking at wanting to stay sitting back. although our bodies are going to have the tendency naturally to want to sit forward due to the heavy weight you're squatting, pulling back can alleviate some of the stress off your knees.
Adequate warm up isnt a factor as you said you do warm up, i'd definetly give it a go with other squat exercises. Hack especially, but that's just MHO. Speed sets like wilson said, the 20 will kick your toosh! Give it a go, it's awesome. I ass to calf squat heavy every 6 days as well, and normally pyramid them. When i have an inkling that my pains are displaced, knees, lback, hips, i fine tune my leg work out to muscle specifics. ie, leg ext, leg curls, abduct/aduct work, with grueling drop sets to keep them firing. Do that for 2 leg workout weeks then go back. Muscle memory rocks girl, and we've gotta be kind to those mugs. Hope any of this helps,
happy squatting,
train hard
~g~