[3.5] advancing a monster's spell resistance

[IlPazzo]

How does a monster's spell resistance advance? There's a guideline for 11+CR for creating new monsters in MMI, but I can't find anything about advancing an already existing monster.

[wotmaniac]

The best I can figure, unless it says otherwise, monster SR is generally based on CR; but this gets dicey, at best.  So, if you're advancing a Bearded Devil 4 racial HD, that should be a +2 to the SR, to correspond with the +2 CR from advancing those 4 HD.  Once you start tacking on templates, that's a different story.

Creatures that have SR based on HD will say so in the entry (the first example I can think of off the top of my head is a pixie).

How does that answer your question?

[ketaro]

Alternatively the Epic feat Improved Spell Resistance for +2 SR. Take multiple times for stacking effect.
Not that hard to say that this is clearly not suitably Epic material and thus can nix the Epic requirements.

Monsters tend to have excess of feat slots and not many decent feats to take
Why else do so many monsters stack Toughness!

[kitep]

It's a single example, but a Thoon Elder brain (MM5, p125)

Advanced Thoon Elder Brains
If you add Hit Dice to a Thoon elder brain, increase its spell resistance and caster level by 1 for every Hit Die you add.

[IlPazzo]

The best I can figure, unless it says otherwise, monster SR is generally based on CR
[...]
Creatures that have SR based on HD will say so in the entry (the first example I can think of off the top of my head is a pixie).

How does that answer your question?

Pixie's SR advance by 1 for every 1 class level (not racial hit die), which also equals to an increase of CR of 1, so that actually seems tied to CR.

It's a single example, but a Thoon Elder brain (MM5, p125)

Advanced Thoon Elder Brains
If you add Hit Dice to a Thoon elder brain, increase its spell resistance and caster level by 1 for every Hit Die you add.

I don't have MM5, but by that wording it seems to be implied that this kind of advancement is an exception.

So it seems to be CR+bonus, where the bonus can be found assuming the formula to be true for a standard monster of that kind.

Thanks!