I can't use this method. I've scars and sensitive skin after a mastectomy and then menopause. Soft tissue which will redden and go dry if anything digs in.
I don't feel that these issues are unusual. Granted, many women have not had mastectomies but many have soft tissue, dry and sensitive skin. No matter what size they are. I have tried to build muscles up and have but still got a layer of soft skin over them on my back and underarm area. I'm not big - the opposite 5 ft 7 and size 8 with a BMI on the low side so losing weight would not be the best option for me.
So I choose bras with a 34 back size. (Measure between 30 and 31). However, this seems almost taboo now. They don't ride up and don't leave red marks. They are the most comfortable apart from comfort bras/bralettes (which I get in a medium). Tighter bras leave red marks and my skin gets dry.
So sick of hearing 'You'll get used to the underband being tighter' and 'You need to go up a cup size' . Complete with a Facebook page. I've tried it. It doesn't work. I see so many pictures even in lingerie catalogues of flesh billowing over underbands. Underarm rolls and back rolls (Although the breasts may defy gravity). These rolls aren't suggestive of a good fit. Granted my breasts are not as high as they could be versus hoisting them up with a smaller band and shorter straps. They don't look like they are about to defy gravity. But they look normal and don't sag, when wearing my bra, at least. They're not at my waist or anything - they are where they should be.
I understand it might be empowering to get some uplift / look slimmer but horses for courses. Running everyday, watching what I eat, walking and weights got me slimmer already. My bra just needs to be comfortable, hold a prosthesis and not show under my clothes.
But such is the Boob or Bust evangelism that many stores will not even publish measuring guides. Thankfully, I do know about 'sister sizes' - one thing I can thank Boob or Bust for, I suppose.
Rant over. 🥴