Bra measuring help

[Chimneyissues]

I have lost weight, which mostly all seems to have gone from my boobs. Today no bras seem to fit.
Are there any good guides for self measuring. Problem how do you self measure when your boobs are flat? Do I stick to my old size with a more supportive shape?
I have time to pick some up in Asda tomorrow but not to try anything on so want to be vaguely correct.

Measure tightly under the bust to get the back measurement

Measure more loosely over the fullest part of the bust (some guides also tell you to lean forward at an angle and measure around the fullest part of the bust)

The different between the under and over bust measurements gives the cup size. Each inch difference increases the cup size. 1 inch = A 2inch = B, then its C, D, DD, E, F, FF, G, GG, H, etc