When doing leg exercises, where you place your feet is of the utmost importance. The main reason being that we are so individual in our stance. So much so that the government has developed a method of identifying us by our walk. It is said to be as accurate as fingerprinting. There are so many variables in the entire pelvic girdle, including the hip bone, sacrum, spine, pelvis, the legs, knees, ankles and feet as to make for an almost infinite number of possibilities. No one can look at you and tell you the correct foot position for you without a thorough knowledge of all of them. The only criteria is how you feel. Comfort is your determinant. Get into the machine or if you are doing squats take an empty bar on your shoulders and do not look at your feet. Look at a spot a foot or so above you on the wall in front of you. Find a comfortable foot position and slowly go down into a squat, noting all along the way how everything feels. Your balance, feet, knees, hip joint, ankles. How does the bar feel on your shoulders. Are you comfortably balanced in the bottom squat position. Adjust your feet while you are in the squat position. Come back to standing. Do you need to readjust your feet? Repeat this over and over until you are comfortable in every phase of the exercise, standing and in the squat. Replace the bar in the rack and start from the beginning. When you can take the bar off and find your foot position quickly you are ready to add some plates.
When I was Olympic lifting we used the squat style to snatch, and clean and jerk.. It is critical that you sit up almost but not quite vertical in these lifts. Most people can not full squat and keep their foot flat on the ground. There is a tendency to raise the heels and sit on the balls of the feet. So the lifters add extra heels to their shoes in order to get the proper angle of the foot to enable them to squat and keep all of the foot on the ground. This, also, by allowing you to sit up more vertically takes a tremendous amount of strain off the back. Depending on the shoes you wear, you would be wise to find a block of wood which when placed under your heels would enable you to sit perfectly balanced in the low squat position. This may be anything from a quarter inch to two inches. You do not need the block when using the machines because balance is not a factor. But you should use the same procedure to find your correct foot position. The secret is, do not LOOK for a foot position, FEEL for it. Close your eyes and your body will tell you everything you need to know.