Never had a Can-Am or Marzocchi forks, but for the conventional forks I've had a good rule of thumb is to fill up to 4" from the top of the tube, with the springs out and the forks fully compressed.
Oil level mainly influences bottoming resistance / stiffness at the very end of the travel. If you are not using full travel, take some oil out. If it bottoms too easily, add a little oil. My guess is 4" from the top as measured above will get you in the ball park.