What would you think of a fellow engineer who suggested the use of 3/4-in. tubing to carry 250,000 Btuh from a boiler to a large floor heating manifold station located 200 feet from the mechanical room? You might do some quick math using an old rule of thumb that equates 10,000 Btuh of heat transfer to each gpm of water flow. Based on this you'd probably conclude there's no way a 3/4-in. tube could carry the necessary 25 gal/min flow. Perhaps you'd then tell your colleague to think in terms of 2-in. rather than 3/4-in. piping.
Rules of thumb are good for conceptualizing a design, provided all the assumptions they are based on are appropriate to the situation. In this case, equating 10,000 Btuh with each gpm of flow is based on an assumed temperature drop of 20 degrees F between the supply and return piping. However, who says that all hydronic distribution systems must operate with a 20 degrees temperature drop?