As a starting point...
...you can start with 10% of the breaking strength of the cable and move up to 15% after the first adjustment. Then go for a test sail and check the leeward shrouds at 15kts. If they stay firm you're probably done , if not bump the loose ones up to 20%. Of course, mast rake and prebend must already be in place.PS a Loos Guage is a must for this job