That's actually a bit funny - with arabs, it's a longer cut, with quarter horses it's shorter! What type of showing are you doing? Are you taking him to a Quarter Horse show, an Arab show, or just an all breed show? If it's an all breed show, the ear rule you speak of is perfect.
If it's a Quarter Horse, I'd do just a bit shorter - just enough to get the bridle on with maybe a centimeter left out. For an Arabian show, I'd do maybe an inch or two longer than the ear :)
I would pick depending on what you think your horse exhibits more primarily in the neck and head region... quarter horse or arab then go with the breed standard for that... Or since it's an open I don't see why you couldn't get away with a regular 3 inches or so... Personally I would just go with the quarter horse standards because I for one HATE cutting my horses mane/tail... Good luck!