[TowerTalk] screw anchor depth

[bob finger]

I have tried to find the answer to the following question in the Rohn 
literature, to no avail.  Perhaps one of the towertalk "experts" can 
provide practical advice.  How deep does one need to screw in a Rohn 
GAS604 screw anchor to achieve approximately the rated capacity?  The 
anchors themselves are 46 inches long in total.  It would seem to me 
that about 12 inches out of the soil would be about correct.  Anyone?  
thanks.  73 bob de w9ge