Adjusting Your Chair
Adjust the angle of the backrest, and/or the length of the bottom-cushion, so that your back is supported in your work position, and the bottom-cushion is not too long for you.
If you can't sit fully back and upright without the back of your knees hooking the front of the seat, you need a different chair with a shorter bottom-cushion. Adjust the backrest height so that it supports the lower back.
Many people set the back support too low. Sit very upright. If the shape of the backrest does not fit fully into the small of your back, you would probably benefit from adding back support.