Out of curiosity I applied your formula to the total volume of my house using 5 as the multiplier. It worked out at just under 50,000 BTU (≈15kW), which was the size of the original boiler. But since the house was built, about 30 years ago, the house insulation has been improved considerably - walls, windows and loft - so the heat loss is now ≈ 7.5kW - half what it was originally.
Applying your formula to a modern well insulated house will result in radiators which could be twice as large as is necessary, which is good for radiator manufacturers and also allows the system to be run in condensing mode even when the outside temperature is well below freezing. But you don't need to double the size of a rad to achieve that.