You should tune you shrouds to 15%- 20% of the breaking load.
Here is how you do it:
Start with both cap shrouds hand tight (using the turnbuckes of course)
Use a 2 meter or so folding ruler and tape the ruler to the shroud
to that the bottom is a few centimeters above the swage fitting. The distance is not critical... BUT you MUST measure what it is...
Now tension the shroud
with 2 wrenches and as you do it will lengthen as you tighten it. The distance from you measure above will start to INCREASE. When THAT distance has INCREASED by 1.5MM (you need a vernier caliper to measure this small distance)
Now move to the PORT shroud and tension this shroud and check the distance on the STARBOARD shroud. When it reaches 3MM you have tension both shrouds to the right amount.
You can use this meathod for any rigging
1x19 wire 5% of breaking load for each 1mm of stretch
Dyform wire 5% of breaking load for each .95mm of stretch
Rod 5% of breaking load for each .07mm of stretch
Tension lower shrouds to keep mast "in column" - straight.