Simplisticly... 20% more area than the flat horizontal and then mount the halves at 110 to 120 deg from each other. (120 is easy to hit exactly. 110 is better)

It works.

I THINK the ideal is 110 deg... can't remember. It will fly anywhere from 90 to 130 deg. If the V is too narrow (90) then elevator function acts a lot like an airbrake. If too wide (130) the rudder function gets poor and you have trouble getting out of a spin.