Example:

On my new pattern plane I have servos on the stab. I want to use futaba little round servo arms, which give a servo arm length of about 8 mm. I want to have 25 deg of maximum surface deflection.

The required control horn length, from hinge line, considering max servo angle = 60 deg:

l_2 = 8 mm * sin ( 60 deg ) / sin ( 25 deg ) = 16,4 mm