I don't think you can say how much differential to put in independent of how many degrees of aileron throw the plane has. Instead, starting with equal up and down movement, pull up to a 45 deg up-line and apply full right aileron. If the plane walks to the right, you have too much down on the left aileron. Conversely, if the plane walks to the left, you have too much up on the right aileron. Repeat the exercise for a left roll.
Also, if you have not sealed the aileron hinge line, doing so can help increase your roll authority.