Not to be difficult (or, no more so than is polite) and clearly with my /a/s/s/ logician hat on: I think both of those examples are stronger than you mean. Because, as you know Bob, "iff" doesn't just mean emphatic "if", it means that the if/then lines of implication go both ways in the sentence. And in "If he doesn't start spraying before I have his balls lopped off, then he'll be a good pet," the implication is that not spraying is constitutive of being a good pet, rather than merely a necessary condition of it. Or, to put it another way, the logic of that sentence is such that if the second half is not true then the first half *must* not be true. I.e. If he's not a good pet then he must have started spraying before you had his balls lopped off. But I bet you think there are other ways of being/becoming a bad pet.

I'm sorry. It comes over me like that. I'll shut up now.