Dellcom...
Borrow or buy Loos rig tensioning gages that fit your rigging. Specific numbers aren't used, but a percentage of the wire's breaking strength (usually about 15%) is the usual guideline for tensioning the standing rigging using one of these gages. These gages will show the tension on a scale (the Pro series can be left in place on the shroud while a turnbuckle is adjusted and the tension read).