- Divide listed magic resistance by 5 and add 12, so mind flayers become MR 22. Note this on your stat block/map key.
- When a spell is cast at a MR target, the caster rolls 1d20 and adds their level. If they equal or exceed the above number then the spell succeeds.
- For dæmons, add 13 instead of 12 to the creature's score but also add spell level for their attacker.
Other issues that come up with magic resistance are:
Does magic resistance prevent area of effect spells from working? In the general sense I would say no, while an individual monster may be unaffected by a fireball its companions will have to look out for themselves.
The exception is when a spell is specifically directed at a monster. In those cases, if the spell does not affect the target then it is completely cancelled. This would potentially include lightning bolt and fireball, as well as meteor swarm, darkness 10' radius or silence 15' radius, and so on, depending on the intent of the caster.
Does magic resistance dispel magic which is ongoing? This is tricky because the example given in the Monster Manual of shattering a hold portal spell (obviously based on the scene in the Lord of the Rings [the book, not the film]) simply refers to "certain" spells and does not define them.
I suppose like many DMs I've flip-flopped on this over the years but nowadays I would generally say that if a magically resistant creature encounters a permanent spell against which it makes its MR roll, then the spell is negated. So, walls of iron or ice as well as hold portal and wizard lock would be negated as would, somewhat more controversially, a glyph of warding. A wall of force would simply allow the creature to pass through, sealing behind it again, since it is not normally a permanent spell.
Even with this, some spells are not treated this way. Dig moves earth in a permanent way not because of on-going magic but because the earth is now in a different place and simply remains there, for example, so its effect is not subject to magic resistance at all. Also, of course, the magical effect must be directly interacting with the monster in question; a demon can not cure feeblemind nor cancel the resoration of a lost limb on someone else. Find familiar is not subject to magic resistance.
Does magic resistance work against special attacks? If they are listed as spell-like, then yes. Otherwise (eg, ghoul paralysis, most magic weapon effects) no.
Clerical turning? No.
Is a magic circle affected by magic resistance? No. Protection from Good/Evil is, as it is a general spell effect but the various magic circles which protect from specific beings always work against those beings regardless of MR or anything else.
What is "standard" magic resistance? This is just normal saving throws, which magic resistant creatures also get.
What would "none" mean, then? A creature with no magic resistance would get no saves against magic. This is the suggested method for handling Gamma World characters encountering AD&D spells on DMG p114. I used this system for the time my main group went to the Car Wars universe too.