Stuart Firth
I’ve just been reading some old MRJs where this was discussed. Joe Brook Smith wrote a letter in no. 104 stating that ballast is usually 3/4″ – 2″. General consensus was that the manufacturers always make it too big, so N scale would be more likely to be correct for 4mm. A long time since I bought any but I think I used the finest one that Woodland Scenics made.