Martyn,
A good rule of thumb is that the average angle at your elbow should be a little more than 90° [obviously this angle will vary during the stroke of a file or saw], and your posture should be reasonably upright.
Various tools and activities therefore dictate different heights [as mentioned earlier]
To avoid leg and back fatigue it is best to not stand rigidly, and it can be helpful to stand on a flexible mat which induces small movements.
MichaelG.
.
P.S. … The 'rump rest' [see fig. 60 here] is an excellent alternative to perching on a stool.
Edited By Michael Gilligan on 25/05/2016 18:00:00