Facing a lose-lose question in a deposition

I spend a lot of time with plaintiffs lawyers. This is a deposition question I am hearing lately, especially in cases where  an investigation of employee misconduct leads to discipline: What grade would you give yourself on how you conducted the investigation? There are variations, but you get the idea. If the witness answers A, then the plaintiffs lawyer says, "So, you are perfect?" The witness looks silly at best, arrogant at worst. If the witness answers B, the plaintiffs lawyer says, "So, there was room for improvement ?"(It reminds me of a joke comics told in the Catskills: A mother gives her son two shirts for his birthday, one blue and one white. He comes to visit her the next day wearing the white shirt, and she says "What's the matter, you didn't like the blue one?") You can't win — or can you? How about this as an answer: "I cannot give myself a grade on something like this. But I did my very best to be thorough and fair, and I believe I was."  It is threading the eye of the needle, but it works.