Official Answers from Chaosium
Stackable Rune Magic (page 9)
When casting a rune spell using magic points to boost, is the strike rank
1 + (number of boosting MP) or
1 + (number of boosting MP in excess of 1)?
The former calculation is from the Red Book of Magic (page 9) and the latter from the Core Rulebook (page 194).
The Core rules are correct, the Red Book of Magic page 9 will be updated to:
27 Jan 2021. This correction will be deleted once the final print and PDF are released.
Do magic points spent increasing a spell’s efficacy (e.g. points spent as part of Sword Trance or Heal Wound) increase the strike rank, or just those added to make the rune spell harder to dispel?
All magic points, unless the spell states otherwise.