In addition to the angle of the footrest, you might also want to consider footrest designs that keep your feet closer together. Finally, consider the best angle for your feet as they rest on the footplate. Recent designs have brought the angle of the footrests closer to the body, putting the legs into more of a perpendicular position. Table legs become less of an obstacle, removing the need for flip-up footplates for some people. You need sufficient range of motion to bend at the knees and ankles to use these closer footrest angles.
Some people like to be able to see their toes so they can tell where their feet are.
If you are conscious of your posture and concerned about your feet being even and flat on the plates, you might want to choose a more forward angle. Those with longer legs might need a more forward angle in order to bring the feet up away from the ground. This option might be more convenient for them than choosing a taller seat height, which can raise the knees too high to clear desks or tables. For people with flexion contracture, where the knee will not open to a ninety degree angle, the feet need to be supported underneath the leg. Some chair producers have an optional footrest that can be adjusted back underneath the chair.