Not being a bra-wearer I'm not really qualified to comment, but watching my wife struggle with this over the years it seems like there's really nothing to substitute for trying on bras yourself.
My wife has tried on all sorts of bras, and these calculators, and went to a local upscale store that specializes in bra fitting, and none of them really substituted for just trying them on and finding a brand that she feels comfortable in. Even the professional fitting resulted in a bra she didn't really like after wearing it for a couple of weeks. When she finds a bra she likes, she buys more of them while she can, and tries to repair them if something breaks.
As an outsider these kinds of calculators and fitters seem well-intended but off, like they can give you a good starting point but in the end it's your comfort. It seems like there's too many factors involved, and too many sizing systems, interpretations of those systems, fabric factors, underwire support issues, etc. to really know.
Measurements are only a starting point. Breasts have wildly varying shapes with wildly different support requirements - not to mention plain ol' personal preference. Every brand will have their own specific construction style and fit, with variations between different product lines and sometimes even individual products.
You can't fit a pumpkin into a coke bottle, even if they have the same volume. Breasts aren't any different, so you need the right bra size and shape.
My wife has tried on all sorts of bras, and these calculators, and went to a local upscale store that specializes in bra fitting, and none of them really substituted for just trying them on and finding a brand that she feels comfortable in. Even the professional fitting resulted in a bra she didn't really like after wearing it for a couple of weeks. When she finds a bra she likes, she buys more of them while she can, and tries to repair them if something breaks.
As an outsider these kinds of calculators and fitters seem well-intended but off, like they can give you a good starting point but in the end it's your comfort. It seems like there's too many factors involved, and too many sizing systems, interpretations of those systems, fabric factors, underwire support issues, etc. to really know.