I've tried a fair few methods and always thought the cold was best for an even controlled finish.
I have a copy of Firearm Blueing & Browning by R. Angier, bought in the 1970's but never got round to trying any of his more advanced mixes. A hard wearing finish is not my first priority as I no longer have any interest in shooting the older stuff, though I did regularly, many years since, when there was no need to list guns on a shotgun certificate but we did have to ask for a letter of permission from the firearms department, to purchase the black powder. I'll give some thoughts to your mix. Thanks.
Mel.