I have several detectors and they do vary in length. I wouldn't suggest a fixed number because of other variables. Firstly, the balance of your machine in normal usage. This is affected by the size of the coil and the weight of the unit housing/battery compartment or pack. It will change when you hipmount the unit as well. And lastly what terrain are you in? This a major factor as well.
If you're on pretty much even ground, try to extend the shaft to it's maximum for your comfort while being able to maintain the coil at a level plane directly above and as close as possible to the ground you are searching. There's also the argument that a shorter shaft allows more control and is preferable for a more thourough search pattern. Again, the shafts are usually adjustable because you will not have the same swing as the next MD'er and may well be in very different terrain. HH!
