Vertical stab area
 

Can someone tell me if there is a formula or theory as to how much area the vertical stab should have? Do you include the bottom of the fuse under the rudder? Any info. would greatly be appreciated. By the way, I'm using this info. for a pattern ship I'm designing. A quick overview, the fuse is 60" long not including the rudder and has a wing span of 64" with 750sq of area with an AR of 5. My stab has 20% of the wing area. Again, thanks and fly safe. :D