I create a minor flaw similar to Weak Magic. The difference is it doesn't halve the penetration total but gives a negative modifier.
What do you think about the "right" value of the penalty?
For the purposes of the flaw I should assign another limit: any target having Int ability has at least a magic resistance of 0 against you.
hmmm sounds like it should be named "Impotent Magic" (problems penetrating) 
sorry couldn't resist. 
Said by someone that goes by the nickname of "boxer" it is quite hilarious indeed 
The basic resistance of 0 is easy to overcome: you assign 1 point to penetration and you already overrule this. Unless you ban the character to have penetration in the first place, that is. Would make for an interesting character concept. At least for the first 10 or so yeatrs of saga. After that, his penetration will be amazing none the less so the flaw will stop being a problem at all.
Xavi
Xavi:
You have to subtract the level of the spell from the casting total.
I referred to the penetration total not the penetration ability!
I was thinking also a 10 but 20 sounds better.
Boxer:
Maybe.
I know you have to substract it. In fact I think it is one of the best changes in the game mechanics of ArM5 IMO.
In your case, this is why as you grow more powerful that becomes a no issue. if you are casting a level 15 spell with a total of +40 casting total penetration becomes a non issue if the only problem you have is that your target has a MR of 0. 
Cheers,
Xavi
to be equal to the cost it would have to be a high value, instead I'd go with a -3(?) to the penetration ability.
Allow negative ability scores.
If you have a negative score and try to use AC's they still multiply but you get a negative number. This could make it VERY hard to cast spells on yourself with out dropping parma.
Consider: mage 1 spell level 20 casting total 22 penetration 4, is an AC to himself so personal spel gets 22 penetration(? no book to look up multipliers now). can get past his parma fine
Mage 2 same stats, but penetration is 4-3=1, so 3(?) penetration.
Xavi:
Yes, this is exactly I want. It became non-issue against mundanes but the disadvantage remains toward supernatural beings.
Thanks Agnar, I forget also the healing.
So I must change the rule. He cannot get the penalty against people who agree to cast on them.
This negative to the modifier is a cool idea but I don't want to limit his simpathetic magic.
Does a -20 modifier sound reasonable with the limitations I said earlier?
Quite massive, in fact. -20 penetration renders you useless against hoter hermetics (no WWars for you, baby) and makes everything but a pixie a powerful challenge.
I can see a dude specializing in spells that circumbent MR here for sure.
Xavi
Why not just use weak magic?
It is relatively easy to get around a pure subtraction while dividing by 2 is always going to make a big impact.
Y/2 ≤ Y - 20 for all Y ≥ 40. So while weak magic will haunt a mage through his entire life this other flaw will gradually become less signficant a few years from character start.
My starting magus is able to provide +14 penetration score with a level 20 spell without focus.
With focus and this flaw common people could be affected, too.
This flaw has some folk tradition background I just want only modelling it more or less.
Yes, there are spells circumvening MR. However these spells are not elegant. When I'm the SG I promise to use similar spells against such players. Does it worth?
Spineless Magic (Hermetic, Major) - the caster's spells are always treated as if cast with the Forceless Casting option, and will never penetrate magic resistance. This also applies to magic items enchanted by the mage.
... would be quite a challenge. But appropriate for a School of Ramius Flambeau.
Personally, I wouldn't do this. Modifiers like this don't scale with the character. I can't think of any Flaws which give a flat penalty to an Art. This is because to balance the Flaw, the penalty would be crippling to a starting magus and small impediment to the archmage. (Virtues don't work like this - Special Circumstances, for example is better for the youngster).
I would think of a different way of limiting Penetration. Perhaps the character can't use Sympathetic Connections to boost his Penetration Total? You might also want to look at Vulnerable to Folk Tradition, a Hermetic Flaw in HoH:S. Basically anyone using commonplace rituals to avert bad luck or the evil eye gets MR against your magic.
I'm also not sure about the final stipulation, that every sentient target has MR 0. There doesn't seem to be a reason for this.
All IMO, of course.
Mark
This flaw has some folk tradition background I just want only modelling it more or less.