Greg: Yes, this is it!

For your quick and kind attention to the point, one must needs recognize that you are a Gentleman, and for the fact that you have revealed so quickly what I sought in vain, your scholarship ever should be praised--

I had thought that it was somewhere in "Love's Labour Lost" at first, then culled through the Henry plays. Despairing on Shakespeare, I thought that it might be mentioned elsewhere in literature, and ended up parsing "THE COURTYER OF COUNT BALDESSAR CASTILIO"
(as translated by Sir Thomas Hoby), which had been partly responsible for the introduction of effront-based dueling into Elizabethan society--This, of course can be found by the curious or the perilously idle on-line, as can the Works Attributed to Shakespeare, from whence the following was culled--


TOUCHSTONE:

O sir, we quarrel in print, by the book; as you have
books for good manners: I will name you the degrees.
The first, the Retort Courteous; the second, the
Quip Modest; the third, the Reply Churlish; the
fourth, the Reproof Valiant; the fifth, the
Countercheque Quarrelsome; the sixth, the Lie with
Circumstance; the seventh, the Lie Direct. All
these you may avoid but the Lie Direct; and you may
avoid that too, with an If. I knew when seven
justices could not take up a quarrel, but when the
parties were met themselves, one of them thought but
of an If, as, 'If you said so, then I said so;' and
they shook hands and swore brothers. Your If is the
only peacemaker; much virtue in If.