Sorry, but I disagree. If you take a look at the HARP rules, you will see that some very few spells are instantaneous there, and it's allowed to have a normal action, e.g. melee combat, in the round such a spell is cast. It's spells like e.g. Deflections and Bladeturn, typically cast in combat alongside other actions. But IIRC you will not find any attack spells among the instantaneous spells from the HARP rules - and for good reasons.
IMO allowing all instantaneous RM spell to be cast with only 10% activity would lead to balancing problems as combinations like e.g. Absolution + Full Parry are too powerful. OTOH having all instantaneous spell cost 50% or 75% activity would render spells like Bladeturn almost useless.
That's why I suggested to have defensive instantaneous spells cost 10% activity and the offensive instantaneous spell 75% activity. If you think that all spells that are in a given category, i.e. instantaneous one vs. normal spells, should require the same amount of activity, then invent a new category "instantaneous defensive spells". You would then have
- normal spells, these would require preparation + 75% for casting,
- instantaneous offensive spells, these would require 75% for casting,
- instantaneous defensive spells, these would require 10% for casting.
But in the end it's up to you. We use such a rules for some years now and it works fine.