[TowerTalk] guy posts math

[Lee George AK4QA]

Does anyone have a the formula for guy posts?  
I have a friend that wants tall guy posts (7 feet) for a 100 foot tower so he can walk under them.  I need to show him the stress that is involved in that as opposed to 2 feet out of the ground.
I've always used the wooden pole rule of thumb; for every foot up you need 3 feet down.
Also, if you have the calculation for the back guy (i.e. earth screws) well my friends, that would be gravy on my biscuit!
All the best and 
73,LeeAK4QA