A lot of places still add 4 to your band measurement. My underbust is 28 or 30, but I was forever being put in 32’s and 34’s. Which doesn’t make any sense – but it took me a long time to realise this! Shops tend to size you to what they stock (M&S) are notorious for it.
So get a tape measure out ladies! Whatever your underbust measurement is, that’s your band size (obviously this will vary from manufacturer – and is just a good place to start).
You’re only supposed to be able to get a couple of fingers underneath your band, anymore than that and it won’t offer much support. In some on my old bras you could get another person in, never mind 2 fingers! xxx