The amount of rake you want depends upon the boat. The idea is to use mast rake to balance the helm. Doing this helps keep the boat sailing faster, since (with the forces at work on the hull and keel and sails) you're not always keeping the tiller at an angle in order to get the boat to go straight. Rake also helps use the weight of the mast and the force of the backstay to keep the forestay tight, so you can head closer to the wind. This is especially important for racers -- hence the hefty rake mentioned for a J/24.
Rake is not the same thing as mast bend: curving the mast so as to flatten the sail. You shouldn't have to worry about mast bend much with a Columbia Sabre. Getting the mast to bend even just 4" would likely be an effort, and in less than 15 knots, probably not any use. Your sailmaker can tell you more about that.
In the meantime, attach your main halyard to a wrench by the hole in the end of the wrench handle and put a yardstick down on your cabintop, aft of the mast. Adjust the forestay (don't forget to loosen the backstay) until the wrench kisses the 12 when the boat stops bobbing from you moving around. Snug up the backstay and go out sailing. Feel any difference on the helm? If there's too much weather helm going upwind, take some rake out. Too little helm, add more rake. It's not Rocket Science, but it should make sailing the boat easier (less fighting the helm) and improve performance.