Thank you for this information, which surely others will find helpful. Turning to pg.1273 of my copy of “Machinery’s Handbook”, I see that DOT is given in 000s of an inch, (0.05335 in the case of 1/2” ,tap drill dia is given as 13/32” - seems a bit tight) The pitch is 0.08333. I take it that, as a percentage, DOT is depth of thread divided by pitch X 100 ? Pardon me, but this isn’t something I have come across before.
From your information, I see that while Whitworth might be suitable for mild steel and even cast iron, it would be less suitable in stainless steel. Is that so?