The shffl uair -> regrab can be tricky to land at lower percents (~35-40-ish). If you're having issues with it then you can do uptilt -> regrab up to 40-ish % instead. It'll do the trick like 99% of the time, but it's a little more susceptible to DI shenanigans and you have to make sure you don't get that weird hitbox that sends 'em to the side.
The chaingrab on Falco is the same step-by-step process but with slightly different (read: higher) number cut-offs. Add around 5 percents to the lower and upper boundaries of each step and you should be fine. The only real difference between the two is that you can't regrab Falco after an upthrow at 0% if he DIs behind you.
Regarding the accuracy of this guide, it's not AR/TAS-tested, but it's been developed over the course of several years by Armada, followed by leffen, followed by me, so the numbers are pretty well-determined (and I use the "~" sign to denote approximations). Kadano mentioned at one point that he was going to test the entire CG but I don't think he ever got around to it.
The only potential countermeasure by Fox that I can think of is to SDI the uair upwards in order to avoid the regrab, but I've yet to see it work against a properly executed uair->regrab. It definitely makes the timing a lot stricter, though.