Agreed Both, it is the tension of the leather that is important, but how to gauge it? The Brooks YT video simply has the guy pushing down on the saddle with his palm to get the ‘right’ tension. And of course the tension in a sprung saddle is more difficult to feel.
There's never going to be a precise level of tension, firstly there's a lot of variation in the leather, X tension on the bolt doesn't mean Y flex in the leather. Then we might all have different preferences, I see people riding around on floppy hammocks compared to the tension I like. Pressing down with your hand might be a good enough measure, though I fond just riding it would let me know when it needed a quarter turn.
Anyway, this reminds me of another reason I'm glad to be off the leather and onto the maintenance free rubber saddles. Not for everyone, but for me they're 95% comfort for 100% of the time, rather than 100% comfort for 70% of the time.