Also, I think I need to elaborate more on capshroud, lowers and intermediate tuning:
before tensioning, the mast must be centered (and raked but that's part of the fore/aft stays and deck-level support for keel-stepped masts). This is done with the water-filled bucket on the main halyard, adjusting capshrouds until the bucket is hanging exactly midships and the capshrouds have some tension and feel equally tight. Now, the same must be done with the lowers, looking up the mast-groove to see if it's straight.
After that, the turnbuckles are loosened again, each the same amount of turns, until capshrouds and lowers are slack and the 2 meter section is marked. I just tape the tape-measures to the shroud
. It's best to first tension the lowers if you have aft-swept spreaders. Each should be tensioned the same amount of turns so check that. Next while tensioning the capshrouds, check on the lowers again! Capshrouds on aft-swept spreaders bend the mid-section of the mast forward, tensioning the aft-lowers and loosening any fwd lowers! Adjust the lowers tension accordingly.
After all this, the intermediates are tuned to feel and you must go sailing to tune them further. This is why it is easier to have intermediates run down to deck-level instead of terminating at spreaders! While on a tack, tighten the leeward intermediate until it's not slack anymore, tack and do the same on the other side. Now look up the mast at the groove again (while sailing) to check if the mast is straight. Correct by further tensioning and repeat on other tack. Now stop sailing and loosen the intermediates again with the exact same number of turns until slack and do the 2-meter tensioning thing but each side must be tensioned the same number of turns!
Very good riggers manage to do all that while at the dock
. They developed an extra sense for it. But they would agree that tuning under sail is better (more precise) but that would add too much to their bill and the competition is waiting to take over.
Finally, every rig is different and imo every cruiser should know how to tune it on their boat. I discovered that my mizzen forestays are too small diameter as they get much more tension than 20% of breaking strength when tensioning the capshrouds!