if you get your chair to a height that keeps your wrists and elbows on the desk with your arms at a comfortable, relaxed angle (upper arm slightly forward, elbows a bit wider than right angles, forearms supported), then you should be able to have feet flat on the floor, knees at right angles, support under your thighs as well as butt. If your desk is too short for that, get a taller desk or something to raise up your keyboard and provide support for your arms. If your desk is too tall for that, adjust your chair to that height anyway and then get a sturdy footrest that you can get to the right height to have your legs as described. (edited)