Butterflylady, that sounds about right. I'm glad that you know what size you need so that you can stick up for yourself and avoid purchasing something that does not fit!
Astrid and Angie, I'm always surprised how many women think they are much bigger in the band/smaller in the cup than they actually are, and also hate the perception that anything D cup or above must be a hyper-exaggerated "porn star" build. Recently a couple of my friends, who to the average eye would have pretty "average" (not "small" or "large") bust sizes relative to their body proportions, were fitted and learned that they should be wearing D or DD cup sizes, and in both cases they were astounded. I had that same "aha" moment about 4 years ago, but it took me a while to start adjusting my band size downward & cup size upwards in order to get better support.
Mo, I think we all have individual comfort zones for the best band size depending on how much support we need, and also the size and shape of our breasts and rib cage. I usually prefer to wear a band size within 1" of my underbust measurement, but I've known other women (particularly those who carry less body fat under their rib cage than I do) who prefer a slightly looser band. I really think trial-and-error is the only way to know for sure. I currently wear a 32G (sometimes a 32H in certain styles), and I could wear a 34FF or sometimes a 34G, but the bras I've tried in the larger band size wear out in just a few months - in my 32 band size bras, I can usually get about a year's worth out of a few pairs.
Steph, I'm glad that you found the videos helpful. I did, too - seeing the vast difference that a larger band size creates vs. a properly fitting bra is a good reminder.