I agree the guidelines are massively vague and unhelpful if you've got a certain thing in mind because these things are subjective, but that's their nature, they can't list out every single scenario that might come up after all.
One of the main things is context, definitely. I've got several shower scenes, but there's enough steam to cover everything, the classic anime modesty steam. I've also got one of the character swimming fully naked but carefully placed not to show genitals. I think the prime time example above it pretty solid. You can show sex, but you can't show it too details, you can show violence, but not gore, you can show nudity but not full frontal nudity. I think on you scale, I wouldn't really go any higher than the soft ecchi/+ but at the same time, you can go up to sex without being explicit (it's fairly common to be honest, a lot of higher rated BL have some kind of sex scene to give you an idea).