You seem to be confusing the CoC with the fare rules (fare rules are available prior to purchase after you have selected your flights -- however, you can't access them directly on united.com post purchase). The CoC is published right on their website and does not vary on domestic fares -- https://www.united.com/ual/en/us/fly...-carriage.html. The CoC (not the fare rules) would have the language for the method for calculating downgrade compensation. However, there's nothing buried deep in the CoC about how it determines the fare difference (I checked it and it simply doesn't exist). This is the only relevant section you will find.

"if a Passenger is downgraded from a class of service and is not re-accommodated in a seat in an equal or greater class of service for which a fee has been paid-, the Passenger may be eligible for a refund in accordance with Rule 27. "

However, Rule 27 does not define how fare differences are calculated. The above language could be applied to cases where you buy a post-purchase upgrade where the fee is explicit. However, it doesn't really address the case where you purchased an FC fare originally.