JLW: Here's some more info...
Extracted from "Live steam and outdoor railroading": Nov 2006.
PLEASE buy a copy to get the whole text and understand what boiler design entails.
Your boiler shell is good for 100psi MAXIMUM internal pressure. (If silver soldered). This means the SAFETY valve (NOT "max working pressure control" valve!) should be lifting at or before 100 psi. - I would set mine at 95 psi as if ever it gets a bit sticky it will still keep me SAFE. The Safety valve should then be blowing - with MAXIMUM FIRE AND DRAUGHT before the pressure reaches 106psi - according to the ASME regulations. (Correct me if I am wrong?). If you have a second pressure relief valve to act as a "pressure control" valve, then I suggest it achieves full BLOW before 99 psi. (It may be lifting at 93 psi?).
Aside: As an Engineer, not a loco driver - I always think the guys who ride around with SAFETY valves blowing are simply wasting steam, ergo fuel, and it's just plain bad practice to drive on the SAFETY limiter. Would you drive around town - or open roads at the MAXIMUM POWER and use ABS for all your braking? - Then "why?" have the boiler fired at MAXIMUM SAFETY?
I steam static models - at the club shows, and the only time the safety "lifts" is either on an annual test or the daily check at the start of each day's steaming - Before I start running any engines.
Sorry, I was on my hobby horse.... and hopefully such remarks do not apply to any readers here. End of sermon.
For strength when designing your boiler, I suggest you plan everything else to be strong enough for over 100 psi (Like the shell) then you'll (probably) have a safe boiler. It costs very little in materials for that tiny extra bit of thickness, or stay, and it will help ensure that no-one gives away a "scrapped" boiler in many a decade. (You would be surprised at how many boilers are really rubbish, if not dangerous!). The extra stiffness of under-stressed material helps the longevity of the silver soldered joints - which can be subjected to fatigue and stress cracking with many heating and cooling cycles of the boiler. - So this is perhaps more complex than just joining a few bits of copper together!
Cheers! - Enjoy the build.
(If I have upset anyone, let's discuss. I can be wrong - my wife tells me often enough!).
K