Set endpoints equal and trim to zero. Servo horn almost perfectly vertical, very very slightly to the inside. Jiggle between subtrim, link length and end points to fit steering rack lock stops so that end points are the same value at full lock.
Adjust subtrim and endpoints again on the track, can take a little while but I have trim at zero, a 4% subtrim and endpoints within 3% of each other.
Not the only way to do it but seems to work well