I just got new spreaders installed on a j24 and was curious on how to measure to set the rake on the spreaders. Also how do I set this...shims, epoxy, drilling another screw, I have no idea please let me know.
I'm not sure I understand your question. The spreaders on my boat are fixed. With a distance of 155cm between tips. What we do do is set the mast pre bend "rake" at 2in at naught on the backstay. That has worked very well for us.
I have never moved the spreader angle on the mast. And have never heard of that being done before. But we may learn something new. Is there a reason you want to change it? What is the advantage.
How to measure?: I recall measuring distance from shroud to shroud at the spreaders. But the way to go is to Google "J/24 tuning guide" and you'll find about 5 or 6 of them from all the sailmakers. They are all similar in setup, and hopefully at least some will make it clear how to measure.
More recent (last 15-20 yrs) spreaders slide over a bar that goes through the mast. The bar is bent to the correct angle. Older spreaders were installed in sockets that were fastened to the mast. There was some slop there, so I believe that any gaps between the sockets and the spreaders were usually filled with epoxy.