A couple of amps won't make much of a difference.
The percentage idea will work to some degree, but in my experience, even a breaker with the same current on all poles will have a slightly different drop across each pole.
If a breaker pole is bad, it's usually obvious. I know this sounds a bit unnatural for an electrician (we're used to being exact), but you get a feel for these tests, and you guess at what the drop should be, based on the current and the other poles or other breakers. If a pole is bad, the drop will be much higher than you thought it should be.
It's a lot like meggering stuff, it's about 80% experience and about 20% scientific fact.