I'm no mechanical engineer or professional rigger, so I'll do everything except answer directly. If you want a different approach, one that might garner a response from a mechanical engineer, post a question to cruisersforum or sailinganarchy.
You can find big mobs of published advice from professional riggers, including Brion Toss and even from Selden Mast. Selden has a neat Hints and Advice document available freely from:
Pages 42-4 directly address the tuning of masthead cutter rigs. It also has the standard message (which I will restate and supplement in my own words):
1. the prime directive is to get the mast straight (no sideways bends, no forward bend) and then to pretension the rigging so the mast is in compression.
2. the function of pretension in the rigging is to avoid buckling the mast in a seaway, when pitching and rolling forces could whip an unstayed mast around.
3. the best test of acceptable rig tune is sailing on the wind in relatively smooth water conditions that generate heel of 20 degrees: the rig is acceptably tuned if the mast remains in column and the leeward cap shrouds are not slack (the test being that when plucked, the shroud responds with something like a musical note rather than a rattle) until at least that 20 degree of heel has been reached. The Selden document also suggests reefing and again checking that the mast is in column and any mast bend is smooth and positive (no forward bend).
4. the idea of the table of standard tensions, and carrying a tension gauge aboard, is that so you can (a) get back to standard tension quickly if you have temporarily disconnected a stay or shroud; and (b) check the tension in new rigging over time. So I recommend almost everyone buys an inexpensive tension gauge (such as a Loos gauge) or becomes familiar with the folding rule technique and marks the stays and shrouds for easy reference.
5. tension in the intermediate and lower shrouds maintains the mast in column (laterally straight) when heeled to wind pressure and also when reefed. So the standard tensions in the table are just a guide for repeatability, not a recipe for getting to correct tension.
Page 32 has an important note at item 8, about the permanent deformation excess rig tension can create on a hull that is not stiff enough. The BCC hull was designed by Lyle C. Hess who also designed the rig. Brion Toss, in his The Complete Rigger's Apprentice, spends time analysing a Lyle C. Hess rig that is close to identical to that of a BCC. So a BCC hull should be stiff enough.
If you're using the equivalent of 9/32 wire all around, the big differences to the wire used by Tom Harrer and I are in the cap shrouds, intermediate shrouds, and fore & aft lower shrouds. That's where Tom and I had/have 1/4" diameter wire rope.
Looking at the specs for 316 1x19 wire, the difference between 1/4 and 9/32 is tiny. Standard breaking strength figures are (as in that table) 6900 lbs for 1/4 wire and 8700 lbs for 9/32.
"Tiny" is relative of course. Quite a few BCCs in N America were rigged with 302 wire, because their owners intended them only to sail in cool temperate water, not tropical or subtropical water where the increased corrosion resistance of 316 is an advantage. The standard breaking strengths of 302 wire are 8200 lb for 1/4 and 10300 lb for 9/32. So when I write 'tiny' I mean that the difference between 316 1/4 wire and 9/32 wire is smaller than the difference between 316 wire and 302 wire, to the point that the standard breaking strength of 302 1/4 is close to that of 316 9/32 wire.
Back to that Selden Mast document (because I did not write an essay plan for this answer), note:
* p. 31 has a table on 1x19 wire in mm; and
* p. 32 explains the folding rule technique for measuring rig tension, a technique you can use to get reliably and repeatably to tension in terms of % of breaking strength without a tension gauge.
BCC 116 Zygote,
Scarborough Marina, Moreton Bay, Queensland, Australia