Glenn Ashmore wrote:

Just verifying my standing rigging specs before approving my order and
need someone to confirm my calculations. All the formulas I have found
for standing rigging loads are based on RM@30 but all I have is RM@1. The
formula for RM is RM=Displacement*GM* SIN but I don't have the metacenter
height (GM) either so working backwards;
....
Am I on the right track?

I would guess so. I have been speculating along the same lines while
dreaming of my little schooner. But I've also done some reading, which may
be of help here.

Brion Toss writes in Riggers Apprentice (p. 135):

Righting moment starts at 0 with the hull upright, then climbs in nearly
straight line to at least 30 degrees, sometimes up to 40 degrees. After
that, RM increases more slowly, to its maximum, and then begins to
decrease. Since the maximum RM indicates the maximum sustained load the rig
will have to bear, you'd think that designers would use this figure as the
basis for mast scantlings. But this is not so; they find the RM for 30 or
40 degrees, depending on the formula they are using, then multiply by a
safety factor to take maximum RM into account. Why? For one reason it is
very easy to find RM at a small angle of heel, then extrapolate along that
nearly straight line to find RM30 or RM40, then work in the extra factor.

In the Nature of Boats, Dave Gerr writes (p.293):
Again, there's a simple rule of thumb. for cruising boats, the total
breaking strength of all the shrouds on one side of the boat should equal
about 1.1 times the displacement of the boat. Racing vessels and day
sailers can get by with the total exactly equalling displacement, while
serious off-shore cruisers should have a total shroud breaking strength of
about 1.2 times the displacement.

(But count only one of the double lower shrouds, he warns!)


This should act as a good sanity check for your calculations.

Regards

Heikki