Having issues with Bra fitting - 30G UK vs 28GG

Measurements:

Loose under: 29

Snug: 28.5

Tight: 28

Standing: 36.5-37.5 (about a week and a half before period they get bigger)

Leaning: 38.5-39

Lying: 35.5-36

The calculator gives 30G UK, so a 30I US. I have yet to order one, but a few months ago when I was around 6 pounds lighter, it was too big for me. So I wore a 30FF. The band was comfortable but was a little loose and moved a bit. So I downsized to a 28GG/J when I noticed the cups were a little small.

The 28J is tight though (doesn't move around), but I feel like the 30I might be too big? I also tried on my friends sister size (she is a 34G US) and the cups were huge but I think that was the band? (we were switching shirts and I needed a bra for the one I was gonna wear, idk).