1 You should sit evenly on both sit bones with your thigh supported by the seat of the chair
2 You should be able to place both feet flat on the floor
3 Your knees should be slightly apart in order to support your weight, and your feet should be under your knees
4 The back support should be a high as your shoulder blades.
Here is an workplace assessment you might light to try.
Details on Care for your Back Course starting 12th
This information was copied and paraphrased from the following
Facilitated Stretching – R E McAtee and J
Stretching and Flexibility – K Laughlin
Overcome Neck and Back
Pain – K Laughlin
PilateSystem – T Blount and E McKenzie