If I use static damage from monsters, how do I calculate the value on critical hits? I can't just double the value, because that includes the static modifier. Should I recalculate the damage on the fly (so, if the monster usually has (7) d8 + 3, I make it (11) 2d8 + 3)? That's kind of pain in the ass. Or do the monsters not double their static damage on critical hits and only get the benefit of the guaranteed hit? Alternatively, I could decide that I roll on crits.
Are there official rules the govern this?