Simplifying Magic Resistance for B/X

In furtherance of my new d30 project mentioned in recent posts, I'm working on a demons encounter table. As a fan of Tom Moldvay's approach to creating a clear hierarchy for monsters with different strengths (a la Basic's dragons and many of his creatures in Lords of Creation), I'm attempting something similar as I adapt (and expand) the demons from the various AD&D resources. 

Something B/X doesn't really accommodate in its RAW, but I'm also not willing to fully abandon, is the concept of Magic Resistance as a stat for demons (even at very low levels/Hit Dice). 

Eric Diaz over at the Methods & Madness blog wrote about this a couple of years ago. I agree with Eric that only a few creature types should have this kind of magic resistance, and I do generally like the suggestions he's made for handling this. That being said, neither quite fits my personal style or house rules approach, so here's what I've come up with...

Magic Resistance

Some creatures (most notably demons and other supernatural entities) possess an innate Magic Resistance that disrupts spellcasting around them. When a spell is cast upon such a creature, there is a chance the magic simply fails in its presence.

Essentially, Magic Resistance (MR) permits a “preemptive” Saving Throw for a creature with this trait against magical effects. If the MR check succeeds, the creature ignores completely the spell or effect; if the check fails, the creature is affected normally by the spell or effect.

To resolve Magic Resistance, start with a base target of 10, add the caster’s level, and then subtract the creature’s Magic Resistance rating (typically 1–20) to determine the final number needed on a d20 roll (made by the DM for the creature). If the roll result meets or exceeds this number, the spell or effect fails against the creature, otherwise, the spell or effect affects the creature normally. A spell or effect with a target chance of 1 or less will always fail against the creature and a spell with a target chance or 20 or higher will always succeed.

• Cast Against/Direct Interaction: MR functions against targeted spells (e.g., charm person, magic missile) and, if successful, negates the spell entirely.
• Area Effect: If MR succeeds against an area effect, the resistant creature inside the area can ignore the effect entirely, though it affects others around them normally unless they are also magic resistant.
• Proximity: MR does not affect any spell simply because it is cast nearby; the spell must target or affect the creature directly.
• Environmental Magic: MR is not effective against indirect magical consequences (e.g., Earthquake causing a ceiling to fall, or a magical charm causing a rock to fall), only the direct energy of the spell.
• Duration: The resistance is innate, always active, and even applies to beneficial spells (e.g,. even a cure light wounds spell cast against a magic-resistant creature would require a check to see if the spell succeeds, regardless of the wishes of the magic-resistant creature).
• Ongoing Effects: For spells or effects that persist or repeat (e.g., cloudkill, wall of fire), Magic Resistance is checked once per round of exposure; a successful check negates the effect for that round only.
• Magic Items: Magic Resistance also applies to spells or spell-like effects produced by magic items (e.g., wands, rods, staves, scrolls). Instead of using caster level, the DM should apply a flat adjustment based on the item’s strength: weak items (e.g., most wands) are treated as +5, intermediate items (e.g., most rods) as +10, and strong items (e.g., staves) as +15.
• Standard Saving Throws: Even if a Magic Resistance roll fails, the resistant creature is still permitted any standard Saving Throw to avoid or adjust consequences of a spell or effect as prescribed by that particular spell or effect (e.g., if a Magic Resistance roll fails against a hold monster spell, the resistant creature is sill permitted the standard save vs. spells to avoid the effect).