I really agree with you underbust=band size. DON'T ADD INCHES!
That being said, if you're in a really ill-fitting bra right now your bust measurement might be off. So, what's the solution? Take your bra off, bend over so that your upper body is parallel to the ground and measure around your bust, over your nipples. It'll give you a much more accurate measurement than just measuring over a poorly fitting bra.
For instance, I'm still waiting for my new bras to come in the mail but if I measure with my old bra on (that doesn't fit at all) I get 41" around my bust. If I take that bra off and measure with the method mentioned above I get 43-44" that's a pretty big difference!! Now if you have bras that fit you more or less well you should get roughly the same measurements for the two.
Here's how to know if a bra fits you well:
Signs of a poor fit.
How to put on a bra correctly
Signs of a bad bra fitter
Why are many measurement guides wrong?
Are you wearing the correct bra size (with pictures that show what fits and what doesn't)
Bra fitting 101
I talk a lot about bras on my blog and finding clothes that flatters a larger chest (a major issue for me) so this issues touches close to my heart.
There is also an AMAZING database that reviews bras and you can put in your measurements and get recommendations. It's still relatively new but definitely try it out! Bratabase
ETA: Remember this is just a starting point!! You really need to try on bras and see because depending on your breasts shape you might have to go up or down a cupsize and brands vary too.