The curve of the backrest should support the natural curve of your back. You should not feel too arched, nor should you feel unsupported.
Armrests, if you have them, should adjust to match your elbow height when your elbows are relaxed at your sides. If your armrests are too long or too high, they will interfere with proper keyboarding position. Your armrests should allow you to keep your elbows relaxed at your sides and should not interfere with access to the work surface If your armrests do not adjust for proper fit, consider removing them.
Your feet should rest comfortably on the floor or, if necessary, should be supported by a footrest. For most people the knees should be at a level equal to or slightly lower than the hips. Adjust the chair height to attain a natural inward curve of the spine and optimize the comfort of your lower back. If the chair is too low, your lower back will flatten or round out. If the chair is too high, your feet, and therefore your back, are unsupported. Circulation to the lower leg can also be compromised if the chair is too high.
There should be 2-3 inches between the back of your leg and the seat of the chair. This will allow for a natural bend in your knees. If the seat pan is too short, it can create pressure points and discomfort in the back of the thigh. If the seat pan is too long, you will not be able to sit back in the chair. Some chairs have adjustments that shorten or lengthen the seat pan if necessary.