I seem to have the same dilemma trying to set up bindings on my new SL.
There is no reference point or default holes.
Setback apparently should be only around 0.5 to 0.75".
But for me it is more than 1.5" now.
Counting from the middle of the board I used the 2nd and 4th screw hole in the front binding.
And the same - 2nd and 4th from the middle on the back binding.
Distance is nice 21". But the setback seems 1.5" so too big?
Refer to above posts shall I move back binding up to the front by one hole.
Seems like it but again it makes little sense and I am supposed to carry a measuring tape to work out the correct holes?
Maybe I am being stupid but never had this problem before as there were default holes marked as reference.