Spell Resistance Houserules

[Dastis]

My dm decided to houserule spell resistance and I'm having trouble deciding if I like it. As the parties primary caster he did say that he would change it back if I hate it too much

Creatures with spell resistance gain a bonus to ac against spell attacks equal to 1/2 their spell resistance. In addition they gain 1/4 of their spell resistance as a bonus on saving throws against spells. No roll is required to overcome spell resistance. Spell Pen allows you to ignore 2 points of this touch ac bonus and 1 point bonus to saving throws.

How does this affect the probability of my spells landing and is there any major reason I should dislike this change?

[Mike J]

I think the changes are a mixed bag and depending on the situation are either a nice bonus or a really bad deal. I'm assuming the new bonuses only apply to spells where SR applies. If it applies to all spells, there is nothing good and you should go back to RAW.

SR is usually set at CR + 11, giving a flat 50% miss chance for level-appropriate monsters. Against saves, spells land 25% to 75% of the time, depending on the save targeted. This makes SR spells land 12.5% to 37.5% of the time. This doesn't really change with level.

Your GM's new method varies with level, since the flat miss chance is now a bonus to saves that progresses every 4 levels. What that does is makes spells that target a strong save fall into 5% success range (Nat 1 to fail). At the same time, spells that target a weak save start out at a 60% success rate (at 1st level) and fall to a 35% success rate (at 20th level) with the break-even point around 18th level. So, at lower levels with you targeting the weakest save, the new method rocks (except there aren't many SR monsters at low levels, d'oh!). Otherwise, you're better off with RAW.

Spell Penetration is likely a wasted feat with the new method, as anything that increases your spell DC by +1 has the same effect and applies to all your non-SR spells too.

I didn't run hard numbers on AC, but many of the AC-related spells are touch attacks and the better ones also have a save (Necromancy). The AC bonus is punitive and like saves, increases with level. At 1st, it is +6 AC. At 10th, +10 AC. At 20th, +15 AC. That is really steep for a 1/2 BAB character even if it is against touch AC. If the spell then has a Fort save (often a monster's best save), you'd probably do more good using Aid Another to give someone a +2 to hit.

If it were me (I play debuff-style casters), I'd "hate it too much" and go back to RAW.

TL;DR: It depends. New method is better at low levels and when targeting weak save. RAW is better otherwise. AC aspect is bad and gets worse with level. I'd say no to it.

[Distant Scholar]

Edit: Ninja'd. Ish.

The answer is, of course, "It depends."

Personally, I'd try it and see if I like how it feels. The main advantage I see is it eliminates the SR die roll, which probably speeds up play.

My mathematical analysis indicates that against good saves, the change is better for you vs. low-CR monsters, and worse vs. high-CR monsters. Against poor saves, it's the same or worse for you. Against AC it's even harder to say, but the change is probably worse for you vs. low-CR monsters and better vs. high-CR monsters.

Boring Analysis:

Let's make some assumptions:

• The opponent has SR equal to 10+its Challenge Rating (which seems typical).
  
• You typically fight creatures equal to your CR.
  
• Your caster level equals your character level.
  
• Your spell DC stat has a +4+level*2/5 bonus [+4 at 1st level, +12 at 20th].
  
• You always cast spells of your highest level.

Your 1d20+caster level has to match or beat their SR (10+CR). This will happen 55% of the time. SR will cause 45% of what would be successes to fail.

According to the Monster Statistics by CR chart, monsters typically have a good save of about CR+2, and a poor save of about CR-2. Your spell DC will be 10+level/2+4+level*2/5, or 14+level*9/10. Let's call it 14+level. A monster with no SR would typically fail to save against your spell 55% of the time with a good save, and 75% of the time with a poor save.

With this alternate SR, they'd get a save boost of (10+CR)/4 ,which ranges from about 3 to about 8. It's trickier to say, but the monster with SR would fail to save against a spell targeted at a good save about 15-40% of the time. Depending on CR, this means the SR will turn a success into a failure between 30% and 70% of the time [instead of 45%]. For poor saves, the spell would fail about 35-60% of the time, or SR will turn a success into a failure between 45 and 80% of the time [instead of 45%].

Difficulties: It's hard for me to tell whether the percent change in successes or the change in raw number of successes is the appropriate measure for this. I went with percent change, because it was a bit easier to work with. Also, I made a lot of assumptions, and did some rough rounding of numbers in this process.

I don't have a good assumption for touch AC, so I didn't formally analyze it.