i'm about as clear as a stagnant pond sometimes

OK, if you look at the tail or a normal layout you have a upside down T shape, if you put this in a box with the tail at the bottom and the sides at the tips, then the top is at the high point of the rudder. The V-tail should be in the top corners, this makes sure its the right angle and size. it has more area than the tailplane and if you have a high fin the V-angle is greater to give greater stability as the fin would have. A large span tail gives a shallower angle which gives the V more pitch stability.

Now.. how you work out what the normal tail section should be is a different matter.

If you just want to throw something on, make each V 20%ish greater in span than what you would use for each side of the tailplane, then use an angle of 110 degrees ish between the V.
In the past i've done things like this more by looks than using any formula. its worked so far