Yeah, tape measure sucks and really a pain in the butt. Almost 19 months and my breast line is still about 39.5. What has changed for me is the under. Looking at my before and after pictures I can see the difference, and most importantly I can feel the difference.
You would think by now there would be something regarding a bra calculator, but alas, I have yet to find one as well. Best bet is to find a place to get properly fitted, or find a discreet store to try on various sizes for yourself, or best guesstimate with trial and error and hopefully good return policies.