heyland factor is probably used more often in europe or even not given any formal name as such but the derivation is as shown and has been very common for over a century the 'heyland factor' or it's unidentified equivalent is how you derive transformer or motor from short circuit and open circuit measurements which is a classical approach to doing it and which leakage inductance article has been trying to describe incorrectly for several years this stuff is proposed in leakage inductance article not in transformer article proper i am not proposing anything major in difference in transformer article proper right now everything about leakage inductance is nonsense so the alternative is to scrap leakage inductance article but there can be no doubt about the substance of the derivation per se one may quibble with the presentation and the naming of this stuff but not with the substance i will address these issues in proposed changes to leakage inductance article you are the one who suggested to start with ideal transformer and to first with linear cum real transformer so back to the question how do you fill the gaps between ideal and real that is what the 'heyland' derivation answers