At what angle-of-attack (sideslip angle) would a symmetric vertical fin plus a deflected rudder have a lift coefficient of exactly zero?
Here are two links to Tom Speer's homepage. They show the performance of a couple of flapped airfoils. They aren't symmetrical, but it doesn't matter for what you are doing. What you want to look at is the change in the AoA between adjacent flap deflections. 

For instance, it the first link, the there is a series for -3 degrees flap and one for 5 degrees flap. Holding the coefficient of lift constant, you'll see about a 4 degree difference in AoA. Thus deflecting a smallish 20% flap is about the same as rotating the whole foil by half as much.

http://www.tspeer.com/NACA23012/N23012wflap.htm
http://www.tspeer.com/NACA23012/N23012wflap.htm

For a 50% flap, it can be as much as 80% effective. See also Tom's analyses of wingsail performance with various sized flaps at the same site. These are symmetric foils, but you have to do some extrapolation to run the lift down to zero.

Tom worked for Boeing, and has worked on numerous high performance projects including the America's cup wingsails and landyacht projects.