Well, I'm just lazy or else I would've already done it.
But for your testing, this is the way I'd do it. Get my buddy to help me, for starters. Go into training and select Doc and then a computer as some character (Fox, for example). Then, I'd set the computer to human and set his percentage to a certain percent. Then I would down throw or up throw, depends on the character, and test to see when I when I can't hit them. You know, get a range. The fair combos are relative to the chaingrabs as well. Like Fox, for example. You can upthrow chain grab him until around 60 percent. By then, he can DI and jump out before he gets low enough to grab you, but that doesn't mean he can do that in the air. So slowly increase his percentage up until he can start to jump out higher than Doc can jump. Again, you've established a range. Just tinker with it a bit and you've got your percentages for when he can be faired and when he is too low or high to be faired.
That's how I'd do it.