Rules of thumb like this are a good for conceptualizing a design provided the person using them knows all the assumptions they're based on. In this case equating 10,000 Btu/hr with each gpm of flow is based on an assumed temperature drop of 20KF between the supply and return piping to the load. If the system is not going to operate at a 20KF temperature drop, or if the system will use a fluid other than water, Formula 1 can be used to determine the flow rate necessary to convey heat to a load at a specified rate: