Vérification (re)faite, Ch.T. donne lui même la réponse dans The War of the Jewels (HoME XI, The Grey Annals, p. 111-112 - commentaires sur le paragraphe 36):
" Between Menegroth and Thangorodrim on the second Silmarillion map (as drawn: not in my my reproduction, HoME V, p.409) the length is 14 cm, and the scale is stated to be 50 miles to 3-2 cm (the length of the side of the squares); the distance was therefore 218-75 miles, or jsut under 73 leagues [...]. The distance given here of 150 leagues (450 miles) from Menegroth to Angbang's gate, more than doubling that shown on the second map, seems to imply a great extension of the northern palin. The geography of the North is discussed in HoME V pp. 270-2; but since it is impossible to say how my father came to conceive it I discreetly omitted all indication of the Iron Moutains and Thangorodrim from the map drawn for the published Silmarillion".
Discreetly, discreetly, ... c'est vite dit... ça n'a pas échappé à Semito :-)))
Silmo