Jolly Tall
Genghis has hit the nail on the head. It is a very useful and accurate method. During training my instructor used High Key and Low key and thank the lord I didn't have to use it. I've since been introduced to the Constant angle method which has increased my confidence ten fold. I have a photocopy of an old PFA artical that explains it very well. I also have a photocopy of an extrat from a RAF training manual that goes into it in great detail. They are now on to their third or forth photocopy and getting a bit tatty but if you interested email me and Ill send you a copy.
Cheers
H