Do you mean using your formula? I'm looking at the book, and don't see "tight as possible" though I certainly could be missing something. For masthead rigs, he says adjust forestay/backstay to a achive rake, then adjust forestay sag (forestay tension, correct?) by adjusting Backstay. He states that boat designers often choose 30% to 40% of the stays breaking load as Max allowable Backstay tension. He says adjust to 30% and mark as Max., then back off the adjustment to 2/3 of Max. I guess using your formula that does bring the forestay over 30% on your boat. Wonder what "boat designers" would say?
Regarding cap shroud tension he suggests pre tensioning cap shrouds to 150kg, (In one example). "Pre tension acts as a shock absorber in variable wind strengths and the masthead is kept under better control."
Yea that is what I was looking at, don't have the book but as you state he mentions boat designers often go to 40% breaking strength for a backstay. Seems high to me as all the other publications (including ones from mast suppliers) I have read never state to go above 30% breaking. Maybe on a race boat where you don't care if you loose the rig after crossing the finish line (in first of course) 40% would be good. The book does state Americas Cup boats typically load the standing rigging to the maximum possible.