Great question, and one which unstuck me very recently.
Having researched the subject through the manuals it seems that most books say "once in the turn apply rudder to centre the ball".
Not very useful.
I have come to the conclusion, having discussed it and thrashed it out witha number of people, that once established in the turn the amount AND EVEN DIRECTION of rudder is dependent on aircraft design.
I believe older types (very early a/c) suffered from masses of adverse aileron yaw and required OUT OF TURN rudder in a turn.
My thoughts are that it is to do with A/c keel area in front and behind the CofG and especially the size of the fin. that is discounting imperfect rigging of the aeroplane.
It will, therefore, be dependant on IAS, bank angle, CofG position and A/c type.
Being a simpleton I now agree with the books and say that rudder should be used to centre the ball!
That might help a little.