Just a couple of things to keep in mind.
Dont lock out in the extended position, due to most peoples legs differing slightly in length, it can have implications to lower back issue later on.
Position you feet so that they are inline with the knee and shoulder.
Don't exaggerate the angle of the foot and don't place the knee past the line of he shoulder.
Changing the angle of the feet and placing the feet higher or lower on the plate does not place an emphasis on certain areas of the quad muscle.
Position your feet and knee on the pad in a way to enable the same path/movement of the machine you are on.
In other words, work with the axis of the machine.