I’m going to say 30J.
I appreciate your analysis. I am tempted to mention bra size on her YouTube channel - but I'm not sure how to do it respectfully, and not to sound like a pervert.
I was giving sizing women's busts from pictures some thought yesterday. I think you'd need some form of reference to gauge bust circumference - I think a known height is one way.
That’s definitely a tricky one. It feels presumptuous and possibly unkind to point out to her something that’s so personal (and, I’m not sure I would be happy with a man pointing out that in wearing the wrong bra size). Also, it’s possibly a choice on her part - maybe she wants to over-emphasise her boobs with a smaller bra, maybe she’s comfortable wearing that particular size (even though it’s probably not doing her breast tissue or back/shoulders any good).
It’s really common for women to get into the bad habit of wearing a bra that they think is the correct size - often because it’s not hugely uncomfortable. It’s only when they try out their correct size that they feel the world changing. I’ve encountered so many women who have experienced this. So, perhaps not!
But yes - when doing a bra size estimate, it helps to know as much as possible beforehand (especially if it’s not done in-person), such as height/weight - or even the size the woman thinks she is, or the size of bra she is currently wearing. It’s also not an exact science - every body (and boobs) is/are different. But with some experience (having boobs of your own and wearing bras regularly helps), some knowledge and a keen eye, it can be finely honed.