oh dear!
a decent fireman on a fullsize loco judges combustion (amongst other things) by the exhaust emitted from the chimney. i was taught to do the same in miniature, and i have no reason to change this practice after 33 years!
there are significant problems in fullsize these days with supplies of decent coal which changes things considerably.
if anything, the 'old' film of Flying Scotsman especially after Alan Pegler bought it and ran it on BR shows more 'fluffy white clouds' than on other mainline locos in recent years as per youtube clips and personal observation. this is rather pertinant considering that the ER of BR used Nottingham Hard coal usually. this supply ended many years ago.
i could comment a lot further but i suspect it would get rather boring!
cheers,
julian