Welcome johnplum. Great to have a young woodworker amongst all us old farts.
Unfortunately, I can't see your image. Maybe others can, I dunno. The best answer, though, is that it doesn't really matter. Make your dovetails to an angle which suits your eye, aesthetically. Woodworking is full of people who will tell you that there is one right way of doing things, and anyone who differs is wrong, but the reality is that the best woodworkers in the world for centuries took their own view on these sorts of questions, and there is no such thing as right or wrong. Wait for people to tell you 1 in 6 or 1 in 7....don't worry, they're coming up.........but it doesn't matter, really. My best advice would be to open a few drawers in old pieces of furniture in antique shops and so on, and just get a feel for the sort of angles the old boys used back then.