This is technically an expansion, not a revision, of an existing spell. This exists in my Felltower game.
Normally I don't like to multiply by fractions, but Improved Magic Resistance is a +150% modifier, and I'd like to keep the character point cost and energy cost consistent with each other.
As written in Magic, p. 123, except that this does affect using magic items and potions.
Cost: 1 to 5 to cast. Same cost to maintain. For 2.5x cost (round up), the subject gains Improved Magic Resistance (see p. B67)
Item: An Improved Magic Resistance item costs 750, not 300, per point.
Note that B67 says "you may roll against HT + Magic Resistance to resist the effects of magical elixirs" but DF1 says " Targets with Magic Resistance must fail a HT + MR roll for drinkables or fumes to affect them." The specific genre rule - the one in DF - absolutely trumps the general case - the one in Basic - in my game. Must fail trumps may roll. If you want to guzzle healing potions while under Magic Resistance, you need the Improved version.