My bra size measuring guide will usually get you pretty close to your perfect size. But everybody's body is a little bit different, and just a few measurements can't account for each of these little differences. Therefore, the size you get with that guide is a great starting point, and for many people that will be the size you need. However, here are a few things that can affect what size you need.
Cup size: First of all, it can be really hard to measure around your bustline accurately. If your current bra fits badly enough, that can affect the size you measure no matter what. In addition, breasts are... soft, and so it's hard to hold the measuring tape at the right looseness so that it doesn't fall but isn't cutting in at least a little bit. Also, what cup size you need changes based on the shape of your breasts. The basic rule of thumb is that your breasts are very full (like if you are young and have never had kids) you will need to go up a cup size or even a few cup sizes. If they are less full, you may need to go down in size.
Here's the rule of thumb for what cup size is right for you: You want the biggest cup size that you can fill out when putting the bra on correctly and scooping into it. This size will probably seem a little empty at the top when you first put it on, until you scoop into it. This is your correct bra cup size.
Band size: My measuring guide will probably be accurate for your band size. But if you have more padding on your ribcage, that will be more "compressible" and you might prefer a smaller band size than the measuring guide tells you. This is especially true if you are plus sized. Also, if you wear a bigger band size, there will be more stretch to the actual bra band, and so it will make sense to wear a slightly smaller band size because of that, too.
And conversely, if you have an especially bony ribcage, you may wish to size up the band a little bit, so the underwires are not digging into your ribcage. Be careful not to do it too much, though, because you still need the band to support you and you don't want to trade underwire digging in for straps digging in! I have a theory, though, that if only bra manufacturers would put a little extra padding or something under the underwire, then those of us on the bony side wouldn't need to do this.
I'm not going to specify any particular band sizes here, because how bony or not your ribcage is probably is going to depend more on your height, frame size, and body type than and particular band size.
Generally, make sure to try on different styles, brands, and materials, as well. You might be surprised at what you end up liking! If you must order things online, try to buy from a store that has a generous return policy and then try lots of brands, styles and sizes and just return what doesn't fit!
Above all, wear what feels comfortable to you. Be open minded and try things on, but use your judgement and don't force yourself into a size that feels uncomfortable just because you think you ought to, or because it's the size me or anybody else suggests.