1. it would help to define the various types of stays on the mast for us non English readers.This to unsure that we correctly tighten the correct stay.
2. At one point you said you calculated the strength to be 15% of breaking strength. How do you arrive at this figure?
It's GIU !!!! Damn it!!!!
(inside joke...don't worry)
Now, for your information, I have learnt the technical english they use in the United States, so that you know I use (with some difficulty) their terms.
are all cables and/or lines that pull the mast in the longitudinal axis, forward or backwards, such as Fore stay, Stay, back stay, baby stay and Triatic (for mizzen masts).
are the ones that are located on the sides, normally attached to chain plates, and pass thru the spreaders.
As far as the 15%, I said I had calculated it, but it was in response to a question my friend had asked. He sails a heavy cruiser in moderate to heavy winds, and 15% seemed like a good all around value for his case.
Shroud tension should vary between 10% and 20% of wire or cable breaking load. and stay between 15% and 30%.
Also the tension settings should vary with the prevailing wind strenghts the boat normally sails in. 10% shroud tension would be better as far as performance is concerned for a boat sailing in moderate winds, 20% should be better for racers and or for boats sailing in heavier winds, where less flexibility is desired.
In your case, and since I know Malta very well, and you want to race, 20% should be the right setting, and 25% for the stay.