I'm wearing a 38D, the band is HUGER on me, the cups give me quad boobs and I'm nowhere near fitting them. I used the bra intervention thing and it put me as a 40G, I can believe the cup size as they're currently much too small but if a 38 band is huge on me how will going up to a 40 help?
It won't! Those with squidier torsos often need a small band than they measure. In addition, bands with more material will have more elastic and thus more stretch than a smaller band, relatively speaking IYSWIM?
What are your below bust and across bust measurements? 40/49?
If your 38D is giving you quad boob then you need to move up at least two cup sizes (and probably more). On a 36 band that would be a 36F. However, a 36F is designed for someone measuring around 36/43 so that probably isn't going to work. I'd suggest trying a 36G and work upwards from there. Don't be surprised if you need a GG or higher.
TBH, take any daily mail bra sizing article with a bucket of salt. They had one the other day comparing different approaches where a woman had apparently decided she needed the same size following M&S advice as Bravissimo, then marked Bravissimo down because the bra from there, in the same size as the M&S one, felt too tight. Biggest load of bollocks ever. Funnily enough it was also 'discussing' ie advertising optifit