The 25% design note, was a concession to being balanced at the hinge line - difficult and heavy on most models.
somewhere in that range has seemed to be best to prevent the ailerons from ever getting into a "flutter".
Why?
The inertia of the aileron is lower.
On elevators - it is best to hold about the same setup -25%
On rudder - same
Funny thing on rudders - If the total vertical area is "undersized"- such as the DR1.
the tail can wag, as the model simply yaws till the load is peaked - then swing back -then repeats--the rudder can be locked solidly - makes no difference.
Also - on some designs - notably- the old KAOS - the location of the canopy can start the same wag. the air is turbulant from the canopy -and the model starts the same rythmic wag from side to side -
We proved this with small paper cups, taped to the top of a fuselage-
really -
The location of the cups could easily control the wag.