[PnP] called shots

[Melkor]

How do you handle called shots in P&P?

one of my players tried to cut off the head of the big serpent he was fighting with.

If he had succedeed, he would have defeated the creature (48 hpv, CDF 6, OCV 10) in one instant so i feel i SHOULD give a big malus to his "to hit" chances.

i thought about a +20 or something like that on the combat table...what do you think?
there are not rules for this in the official books.

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