A very interesting and rigorous experimentation done by the University of Cassino installing heat allocators in a building in which there were also mounted reference heat meters for each radiator. This research found out typical errors of heat cost allocators around 9% and 40% as maximum. Even worst other heat cost allocation techniques for vertical hydraulic warm water distribution circuits. This graph summarizes the experimentation results:
Typical and maximum errors for the last three allocation techniques in the graph come from University of Cassino experimentation. For a detailed explanation about the heat cost allocators lack of accuracy and the phenomena that could, in average, further decrease the typical error detected by University of Cassino, you may have a read of this article.