You will note in some of my Messages that some of my Converted Monsters (and eventually converted Magic Items) have a Magic Resistance Rating. This is to re-create the 1st Ed AD&D Magic Resistance D100 (%) Roll.
This is taken IN ADDITION to the Monsters Normal Save vs. Magic - and is NOT modified with a penalty to the roll in any way, its a straight D20 roll.
It is expressed as Prime - designated with a 'Pr' (12+) or Secondary - designated with an 'Sc' (18+).
In the case of truly resistant creatures/items, the target Number is also reduced by the Monsters Hit Dice or the Wielders Level; in such cases the letter designation will be followed by a '+' sign.