A good ergonomic chair adapts to the body, instead of in the reverse order. Even when a ready-made model, it needs to feel like it was built for your shape.
If adjusted comfortably, an ergonomic chair feels right when everything lines up. Feet should rest without any elevation. They belong on the floor. A right angle should be at a point where your knees are. Your back should have support without firm pressure spots, and your arms should sit still at your sides.
You will stay comfortable for a longer duration, improve on your posture, and reduce wear from a seat of poor design.
If you are unsure, trust in your body. Don't trust only in specs. The right ergonomic chair for you may help you feel more focused as well as more balanced also less tense.