RE: Spinner size
I would bet that the book actually said... no more than 1/5 the dia... The center 1/5 of the prop is the least efficient area of the prop. This portion, of wood props, typically has a very fat (draggy) airfoil because of the need to maintain strength to keep the blades from flying off.
They were dealing with gasoline engines running an average of 4000 rpm also... Though the engines COULD run higher rpm.. that is where they tended to develop maximum thrust. (one 1939 advertsement notes that an engine was brought up to 9,000 rpm in testing... then the manufacturer's recommended prop for the engine is the size that matches the engine's rated horsepower at 4000 rpm acording to the prop design articles of 1938.)
We aren't as worried about efficeincy either... Our typical .40 size trainer's power to weight ratios are higher than most late 1930's to early 1940's FF models.
Some of the old information is still very good. Some of it is just "interesting." (truth is stranger than fction stuff... like the model with a wing resembling a paddlewheel off a sternwheel riverboat.)