In my experience, if the boat is not a one design class, or a class that is raced a lot, there will be no codified rake measurement. Especially for boats like O'days that are considered cruisers, and not even cruiser/racers.
The best thing you can do is set it by trial and error, sailing close hauled in 10-12 knots of wind, so that you have 3-6º of weather helm at the tiller. When I rigged mine the first year, I had spun out all the turnbuckles to examine and lube with Lanocote. So I had to tune the rig from scratch. At first I thought the amount of rake I had was great, the tiller was so light and neutral. Then it blew a bit harder, and I realized that lightness translated into lee helm. Not good. I tweaked the forestay looser a few turns and the uppers tighter to tip it back and get some weather helm.
It took me about 3 sails until I was done fooling with it. Then I went and stiffened my rudder headstock with solid wood, which caused the foil to kick aft an inch. Tiller got really heavy, and I was back to the drawing board, moving the mast forward again. I might still have some tweaking to do this season, but I haven't been sailing - too much house packing to do.
