The page on my web site is broadly accurate about how I produce the sketches and the limitations: https://www.devboats.co.uk/gwdrawings/howidraw.php . Please note that although drawn to scale I don't claim them to be scale drawings (!)  In the case of these Barry absorbed locomotive drawings I use drawings in Rutherford as well as Russell - and they often disagree on minor details - and refer to photographs as well. For the Rhymney the drawings in Welsh Railway Records Volume 1 are proving invaluable, being of much higher quality than weight diagrams, but I still look at photos and weight diagrams as well, especially when considering variations. If I can find GA drawings these are obviously more valuable than weight diagrams, and I seize on them. I can't possibly afford every drawing the NRM possesses though.
The biggest issue is distortions and errors, especially with the weight diagrams. They've survived decades of sometimes indifferent storage, then been scanned for the publications and the publications scanned by me. I draw a grid with the measured dimensions I have, and I can then juggle a scan so that it fits as well as possible. Chimneys and cab roofs often don't! I treat wheel centres and boiler pitch dimensions as being most reliable. Footplate height is rarely listed in weight diagrams, and for some reason seems problematic. I like to think I'm getting better at these drawings, and I hope am getting a feel for how the original draughtsmen and women drew curves and so on.

With this one the cab was a bit problematic, and I think there were variations. I spent a lot of time with a particularly good works photo, and spent some time puzzling out and drawing how an unusual handrail along the tank top was laid out, only to find, by referring to later photos, that it seems to have been soon removed, so I left it off!