[Elecraft] Where to guy?

[Vic, K2VCO]

I am putting up a Cushcraft R8 antenna on a flat roof that is about 3m x 
3m square. The antenna is sitting on a 1m mast; the antenna itself is 
about 9m tall.

It will have four guy ropes to the corners of the roof. What is a good 
height at which to attach them to the antenna?

Are there rules of thumb for this?
-- 
Vic, K2VCO/4X6GP
Rehovot, Israel
http://www.qsl.net/k2vco/