There are many school of thought on this subject. Some people use the tensile strength of the wire as a starting point and go down a % from that number. While others will set the mast straight with low tension equal for matching wires port and starboard (hand tight). Then go out sailing and sight up the mast track when on different tacks and see which way the mast bends and make adjustments to the rig so there is no flex left and the mast stays straight. This is a tried and true method and the way I do it.
The reasoning is just cranking down on the wires to a number doesn't tell you if its not bending or straight. Also by doing so, your trying to push the mast through the bottom of the boat and it will break the boats back and distort the hull. Many racers use this method but will release the tension after the race. Also you will need a expensive tension meter, not a Loons to do the job. Its not just a numbers thing. The boat is talking to you, you just have to know its language.