Formula for Spell Resistence?

[IcyCool]

This is slightly related to the Simulacrum thread I just opened, but I figure it's different enough to warrant its own thread. How do you determine the SR for a creature? Is it based off of the creature's hit dice (like the Drow)?

My primary concern with this is how it interacts with reducing or increasing the monster's hit dice. Does SR scale with Hit Dice?

 

[Patryn of Elvenshae]

We discussed this last week or so. I'll see if I can't dig out the thread.

Basically, the formula for SR is "A pure caster of level equal to the CR of the creature should have a 50 / 50 shot of beating the creature's SR."

Ergo, for most creatures, there's no defined SR progression, and therefore SR does not progress.

EDIT:

Here you go: http://www.enworld.org/showthread.php?t=143584

 

[Nail]

...in case you're interested:

For MM (3.5e) the average value for SR is CR + 10.

However, if you remove dragons from the data, it's closer to CR +12.

FYI.