From my point of view, an acceptable height for an anvil depends upon what type of forging you are doing. If you are doing primarily heavy blows during forging, a knuckle height is probably optimal. If you tend to use lighter blows, or are learning Uri Hofi forging techniques, or are having arm or back problems, the anvil should be higher. How high is a personal thing. You should experiment. It helped me to move my anvil up almost to where my wrist bends (within an inch or so).