My rule is usually "how pissy am I today?"

That's also moderated by "how much hot water will this get me into", which itself is often moderated by, instead of being a pedagogue, forcing them into defending their ridiculous arguments.

For example: "Wow, it's awfully generous of you to donate that much to charity, even if it is being deducted from your taxes. Out of curiosity, what charities did you want to donate to that the government doesn't allow deductions for?" Yeah, this is a disingenuous argument, but it will either show him to not know what he's talking about or will further the discussion in a, likely, fruitful manner.