I went with 3:2 proportions using Crown Royal and 'Merican Dubonnet (Kentucky's finest, the 2018 new-and-improved version, because I can't find the French stuff, but it's still better than Byrrh). This was plenty of Dubonnet; we both enjoyed it. My copy of Craddock calls for "Rye or Canadian Whisky," which is very different from specifying "Canadian Club Whisky." Many Canadian whiskies, including Crown Royal (which is made in Manitoba, after all), contain some amount of rye, and maybe in Craddock's era he was used to having more rye-heavy Canadians, so the choice of bourbon seems a significant deviation from Craddock's recipe. Does anyone know if the Dandy originated in Canada, before it appeared in Craddock's book?
I agree re bourbon, and I have amended the recipe above accordingly. I've also added to the history, but I think the Dandy first appeared in 'The Savoy Cocktail Book'.