Brake cooling charts aren't suggestions or guesses. They're technical data to which you should adhere. Les, ther are certainly legal implications; fail to adhere to the data provided and you haven't a leg to stand upon. But the same platform which stipulates you must be legal demands also that you be safe. Be safe by paying close attention to your brake energy and cooling data.
I'm not sure I understand the question. The data is there, use it. Period. It is NOT a suggestion in passing; it's put there to be used and followed.
You don't average the wheel assemblies. If the brake limit is 120, and you have a brake at 130, you have a brake over the limit. Do what must be done; put fan on it, delay the flight, depart with the gear down for the prescribed time period if applicable, etc.
If you start your taxi with a brake that's already borderline, the taxi could put it over. You may have inadequate or assymetrical braking in the event of a rejected takeoff. You may find that you end up with a brake fire, or that the airplane won't stop at the next turn in the taxiway. You may end up with brake damage; an hour delay might save forty thousand dollars in replacement parts. You really don't want a wheel assembly to explode or blow out fuse plugs. You have the data before you; use it.