Thanks for the feedback, everybody. With your comments, and input from elsewhere, I think I have what I needed.
I also talked about it with the PO yesterday. He hadn't realized he'd had a brain-fart and had tied the turnbuckles incorrectly. ("What in the hell was I thinking?," or words to that effect, was his response, when I told him what I'd found.) He told me how to get her roughly back in tune. Amongst other things: He told me opposing shrouds are so close to identical, wrt length, that a good start is to match the lengths between the turnbuckle bodies on the starboard side to those on the port side. I'll of course use the main halyard to measure to the main shroud chainplates on each side, and eyeball the mast to see that it's plumb. After that, he said, it's mainly a case of sail her and find out what needs to be done from the way she sails. (Kind of along the lines of what John suggests.)
Somebody on the P30 mailing list also pointed me to a rig tuning guide written by Bill Shaw. I kinda suspect he probably knows what he's talking about
. So I'll add that info to the mix.
Haven't been able to convince The Admiral of the need for a Loos gauge. Doesn't help that most people on the P30 mailing list, the PO, and everybody we've talked to at the club, incl. our sponsor, says "You don't need it."