Squats? how low to go?

i do low bar squats and try to do it with good form

now i would like to know the Difference between hip parallel and knee parallel?

hip is lower but how much

knee i understand is were the top part of your quad is parallel to the ground

how much lower is hip parallel

i think hip parallel is where your hams are resting on your calves but i'm not positive

i know i can;t go ATG with low bar squat so i'm trying hip parallel but i would like to know exactly where that is