I'd be happy to help you out here as best I can, unfortunately it is rather difficult to determine the best size bra for someone online (as you're experiencing!) so if you have the opportunity, I definitely encourage you to consider visiting one of our In Store Certified Fit Specialists-they are very professional and understanding of individual needs. In the meantime, I hope the below can help answer your questions.
First, regarding your question about cup sizes-cup sizing can get really confusing, I always tell people to think of it in the same terms as apparel sizes. For example~when you're buying a pair of pants, the pants get bigger as the actual sizes get higher, so a size 8 is larger than a size 4. Same with cup sizing, as you get higher in the alphabet (or get more D's added to the size) the cup size gets higher. So, if you're finding that that C cup you've worn for years suddenly feels a bit tight, or is causing you to spill out over the top, I'd definitely recommend trying a D cup-and if that is still too tight, then a DD would be a good cup to try.
Since you said you already watched our video, another resource that might be helpful for you is our Bra Fit Guide-you can find it here: http://shop.nordstrom.com/c/womens-bras-fit. The third tab, called Problems and Solutions has some helpful advise on how to problem solve for typical bra fit issues. It sounds like you are measuring for your band size correctly (straight across your rib cage, just under your bust with the tape snug to your body all the way around). Unfortunately there is no way to determine your cup size with the across tip measurement you took. But I can tell you that since you measure 42" across your rib cage, you will want to go with a 42 band. From there, there are a few things we can do to determine the best cup size for you~while wearing your 40C bra check out how your bust fits into the cups. If you are spilling out over the top of the cups (see the picture of our too small fit in our Bra Fit Guide for some visual guidance) you will want to go up at least one cup size, if it is a lot of spillage I'd recommend trying 2 sizes up. Another very common issue women have when their cup size is too small is that the underwire doesn't go back far enough and actually sits on their breast tissue, which is a big reason why bras get uncomfortable after a few hours of wear. Your underwire should go back to the side of your ribs and point toward your armpit. If it doesn't, go up at least 1 cup size. That should get you on the right path to getting you the best fitting bra.
I hope all of this information helps and makes sense, I've also included a few bra options below that I think you might like. The first is our Wacoal 'Awareness 85276' Soft Cup Bra, since it is a soft cup it won't have an underwire and you won't need to worry about getting your cup size 100% perfect, it will allow for a little more "give" in the cup size and still be supportive and comfortable. The second option is our Wacoal 'Halo 65547' Underwire Lace Bra, this one does have an underwire but the lace is stretchier so you will still have a little more wiggle room with your cup size and the lace is actually nice and smooth under clothes.
If you have any other questions I'd be happy to help here or you can contact our Customer Service team by clicking on the "Need Help? Live Chat" link up above. Thanks!