When using hand files, it is best to hold the stock firmly in a vise or clamp. For better results, try to keep the stock being filed at about elbow height. For heavier filing work, the stock should be lower; if the work is finer, try to keep it up closer to eye level.
Don’t rely on eyeball measurements alone as sight lines can often be deceiving. Take a few seconds and use a measuring tape or level to be sure.