Keep your feet flat on the floor when seated and keep your knees in line (or slightly lower) with your hips. Sit up straight and keep your hips far back in the chair. If you can easily slide your fingers under your thigh at the leading edge of the office chair, you’re at the ideal position.
With your bottom pushed against the chair back, you should also be able to pass your fist between the back of your calf and the front of your office chair. If you can’t do that easily, then the office chair is too deep, and you will need to adjust the backrest forward, insert a different lower back support, or get a new task chair.