usually dictates how much effort a person needs to actually get in and out of a chair. Seats that are too high lift your feet of the ground. The lack of lower body support will cause serious discomfort over time.
A seat that is too low will have your knees higher than your pelvic, and likely create problems in trying to get in and out of it, making the user vulnerable to strains.
When standing and facing your chair, the height of the seat pan should be at, or roughly at, your knee caps. . (Note: this may need to change based off your height and the height of the desk but start with this height). Calculate your ideal height by measuring up from the floor and then to the back of the knees. Have someone squat next to you to take the measurement.
There needs to be a 90 degree bend at the waist the entire time.