Angling a hinge line will do a couple of things. If no change is made other than angling the hinge line, the effect of the surface will be lessened. Also, there will be a new direction of the force being created by the surface's deflection.

So you will need to increase the size of the surface if you wish to keep it's original effectiveness. And........

In the case of the rudder, you will need to angle the hinge line to give you a couple in the direction you wish. Since you're trying to counter a rudder couple that causes a nose down pitch, you'll need to have the hinge line angled to cause a nose up pitch. Rake the rudder hinge line to do that. Build the rudder hinge line to be farther aft at the top than it is at the bottom. When that rudder is deflected, it will generate a tail down force.

I would bet that none of the above is any revelation to you. I'm guessing that you were really hoping someone would pop in with a formula that would give you some numbers or an angle to use.

Ain't no free lunch. Truth is, the way most model designers work is from model to model. We build what we think will work and fly it to see. If it works, but works too well, we build another version with less change. If it works, but not enough, we build another version with more.

I would bet that Dick Hanson could give you advice whether or not raking the rudder hinge line is worth the effort. And I'd hazard a guess that he could give you a starting angle, if any were worthwhile.