Aside from the fact that the premise of this article is ludicrous -- a willingness to stay indefinitely in Iraq and to see it as a potential staging ground for a war with Iran (or, as he likes to call it "the Iraq-Pakistan border"), is different not just in quantity (of troops), but quality (in terms of both type of policy, and "superiority in kind"), from a long-standing desire to wind down our occupation -- the formulas do _not_ resolve to the same thing. In particular, McCain's "equation" has a unique answer for Y(X). You have:

140,000 - ( X * 16/X ) = Y(X)

The two Xs cancel out, leaving Y(X) = 140,000 - 16

I rather hope that sixteen months after Bush leaves office, there will be fewer than 139,984 US troops in Iraq.

There's an exceptional case for X=0, which is what happens if McCain fires generals until he finds himself somebody willing to say that withdrawal should be stopped entirely. This would not be without precedent -- the Bush administration amply demonstrated that it could drive out any generals that disagreed with it. In the case of X=0, Y(X) is undefined; quite fitting, for the incoherent McCain, who apparently after nearly six years of study, still can't keep straight the Who, What, or Where of our Iraq misadventure.