I think LAW is fine the way it is. Decreasing by one block would be pointless too, therer barely any difference to knife and machete as long as you go for the head, except maybe against a husk but I havent tried it.
And decreasing by more than 1 block would make demo too powerful.
Just leave it alone.
On harder difficulties knife head shots don't mean much.
Does the Machete's x3 dps vs knife mean anything?
http://kf-wiki.com/wiki/Weapon_damage_table
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