any guidelines for Spell Resistance?

[Misirlou]

Basically, just decide how resistant you want your monster to be.

6+CR = 75% chance to be hit by magic
11+CR= 50%
16+CR= 25%

 

[Dthamilaye]

Of course, casters can have the following to raise their SR rolls:

-Feats (+4 max before epic)
-Spell power from Archmage/Heirophant
-Item with [enchantment] bonuses (max I give IMC is +2, as in robe of archmage)
-Specific powers of PrC (+2 vs outsiders from Incantatrix)
-Caster level +x items (price for slotted item according to IOUN stone = bonusxbonusx15000, max +3 before epic, though +2 is more reasonable)

That means up to +10 to 12 bonus to SR rolls if the caster really wants to get the spells through.