[JohnV]

Brion,
Thanks for weighing in, and addressing the original question. You're right about the numbers.
The DWL is 27.5' and 1/2 beam is 5.335.
I interpolated a RM of 25000, probably high but it's a heavy (#13500, 33') boat.
The formula on pg 136 gives me (pardon the detail):
(25000x1.5) / 5.335 = 7029.
That x 2.5 SF x .45 for the uppers = 7907
That x 2.5 x .325 for the lowers = 5711
I had been reluctant to second-guess the designers, given my own limited knowledge, so I arbitrarily stuck with 5/16" wire and 1/2" T/Bs. And I want to be confident offshore with it; I sail out of Penobscot Bay, Me.
Loos' website table says I could go to 1/4" uppers (302/4) or 9/32 uppers (316), smaller for the lowers. That seems tiny, though I can't support that intuition with facts. It's obviously a huge advantage in cost and weight. It might even make covered Dux affordable.
Do you think I'm being too arbitrary in my original plan? Or too conservative in over-estimating my RM to begin with? I'd sure like to feel OK about looking at cheaper, lighter options.
PS: The chainplates are thru the side decks, so somewhat closer to the mast than 1/2 beam.
John