I've been racking my brain for over an hour and still cannot deduce a formula for calculating a dragon's SR. Heck, it took just as long to figure out the breath weapon save DC!! Formulas you think work at first do not pan out. Any help is appreciated.
I'm not sure how they calculate initial spell resistance -- there may not be a formula. But all (non-epic) dragons seem to get SR (at some level) at young adult age; every age category after that, SR increases by 2.
FWIW, the formula for breath weapon save DCs is on p. 8 of the MM: 10 + 1/2 HD + Con mod.