The bill is not a regulation itself, but merely directs the FCC to create a suitable relation that implements the intent of Congress. So it's not necessary to specify every last detail.

Presumably this would be covered by existing rules and definitions, such as 47 CFR 97.3(a)(8):

https://www.law.cornell.edu/cfr/text/47/97.3

which doesn't constrain the shape of the emitted spectrum at any particular point, but just defines the bandwidth as whatever contains roughly 99% of the total emitted energy.

Measuring bandwidth at the 3db point is the standard measure. The other arbitrary points you mention are covered by existing regulations on spurious emissions.