Once you scoot, then you judge:
If you are slouching in your chair, the backrest needs to be adjusted. Move the backrest forward enough that it supports you in an upright position., while allowing you to lean back against the chair. Move the lumbar support up or down so that it lines up with your belt-line, either moving the sliding lumbar support or lifting or lowering the entire backrest, depending on the type of chair you have. You’ll know you’ve adjusted the backrest properly when you sit straight in the chair, then relax, and you’re not slouching. Comfy, right?
Is your chair right for you?
If you have set your feet on something very supportive, set the backrest to mold to your spine, and scooted your posterior back in the chair, only NOW can you look at the seat and see if it fits!
Are you getting poked in the back of the knee by the front of the chair? If so, the seat may be too deep.
Does the seat end somewhere around your mid thigh? If so, the seat may be too shallow.
Do not despair! Some of the chairs adjust even more!
Take a look at the levers on your chair. Some allow you to make the seat deeper or shallower. If you see a lever marked “seat depth” adjust the depth of your seat to see if that helps.