The spring tension depends on the gauge of the strings, of course. If you are using 9 to 42, you need less tension than say, 10 to 45 gauge. You probably want to start with 3 springs, and the screw plate about 5/8 inch out, parallel to the cavity wall. As you wind up the tuning machines (keeping an eye on a GOOD chromatic tuner), when you end up with all 6 strings in tune, the bridge should be parallel to the body. If above the body, you don't have enough spring tension. If below the body, you need more spring tension.
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Grant