Can anyone tell me what is recommended by the rules when rounding a score? Up or down? i.e. Creo (4) + Aquam (7) in the case of exhausting Spontaneous Magic is 11/2 = 5 or 11/2=6? I see that Aging is fixed to rounding up by the rules, but can't find a statement for spells. I use the rulebook in french, so it might be more obvious in english and lost in the process of translation.

I'm not sure if ErikT's reading is correct, but his proposal would be sunstantially different from rounding down in terms of penetration. After all, 0 MR does resist against a spell with 0 penetration, but would be bypassed by a spell with 0.5 penetration.

Me neither. Personally I prefer to play with mathematical/conventional rounding. I.e. from .5 and above rounds up, anything below rounds down.

This has the advantage that it makes ArM rounding the same as normal rounding, which avoids having to deal with learning a whole new system of rounding just to be able to play the game.

Being both mathematical and conventional of mind, I think it is obvious that you do not round at all, unless the rules explicitly says so. Fractions pose no problem, neither mathematically nor conventionally.

I suspect this "no rounding for spontaneous magic" might just be a 5th ed. thing. I'll make a point of checking next time I've got my 4th core book out, though.