My own inclination is to score each thrust, riposte and counter as if each were an eighth grade writing assignment:

"Vilify something or someone, using any combination of convincing argument, persuasive rhetoric, and sheer bombast.  Correct grammar and spelling is a plus factor.  Fabrication is permissible, but exposed fabrication carries a heavy penalty."

Is this enough?  I'm confident that this scoring method captures some important aspects of flame wars, but I am also sure that there is much that I do not see.  Often, my impression of who is leading and who is trailing is at variance with the impressions of others.  

So, how you YOU score them?

Oh, and please share your understanding of the rules of flame wars, including local variations, grounds rules, etc.