Martin's figures are just the arithmetic for various wire sizes to get to 15% or to 20% of breaking strength, which, as he pointed out is what Selden recommends (15%). The 15% or 20% applies to all wire sizes and it doesn't matter what boat the wire is on. Do note that our SN's have some 6mm stays (marocain and artimon pataras) which has a 6351 pound breaking strength, so to be at 15%, the arithmetic gives you 950 pounds of tension.
You could certainly kick it up to the 20% figures Marin's guy Franck also shows to get to "Amel tight" although I've kept my mizzen backs at 15%. With new rigging you might want to go to the 20% initially to account for the stretch that will occur after some sailing. Nance and Underwood in Ft Lauderdale do an initial tune to 15%, btw. None of this is "rocket science".
Craig Briggs - s/v Sangaris / SN68 Tropic Isle Harbor, FL