If they are full length ailerons just put them about 1/3 of the span not the chord. The reason why not put it 1/2 of the span is: a full lenght aileron experience a slight twist at high aerodynamic loads. That is, given the force to deflect the surface is applied near the root then the aileron tip will measure a lower angle at that point. This is acceptable in our sport. Now if you place the servo, force to deflect the aileron, at midspan then both of the aileron tips will register a different angle of deflection than the one at midspan because of the twist created by the aerodyamic load. That configuration creates a lot of turbulance and unnecessary drag. Think of it as if the wing is a propeller, the higher pitch should be close to the root while the smaller is at the tip. This is to balance out the displacement of air mass while its rotating.