Isn’t it above/below the bust to get the band size, around fullest part to get a number and the difference between the band number and fullest part number is your cup? Up to 6 inches there no triple letters because a difference of 5 is DD. So from 6 on, there’s room to grow sound wise because
6 DDD/EE/F
7 EEE/FF/G
8 FFF/GG/H
9 GGG/HH/I
10 HHH/II/J
11 III/JJ/K etc
I remember Sarenna being on Richard Bey touting 34H, but then caught Ricki Lake a few months later and she said 88GGG. 88 is just the centimeter measurement you get when you divide the inch number 34 by 2.54 (also the speed needed for time travel) So if the difference was 8 between her fullest and over/under from 34 she’d get 42 and be an H cup. But she might’ve also did the round up thing by 1 in order to get GGG. Playing to the audience, she’d know when to bust-out the larger sounding sizes. Even though H is after G, triple G sounds way bigger.
Bra fitting now goes like this:
Underbust measurement is your band size. If it’s an odd number, you round up.
Overbust measurement (across the nipples) minus your underbust is your cup size.
Then, consult the cup chart for your letter. Typically, it goes:
A = 1 inch diff
B = 2
C = 3
D = 4
DD = 5
E = 6
F = 7
FF = 8
G = 9
GG = 10
H = 11
HH = 12
J = 13
JJ = 14
K = 15
KK = 16
And so on. These are UK cup sizes btw and are just general guidance. US cup sizes are slightly different. There’s no triple letters after DDD in US sizing. Sometimes I can wear a 30G, but I mostly wear a 32FF.
I’m pretty sure Sarenna was never a 34 back - she’s too small and slim. And she was very big in the cups, most likely a JJ or a K. So, possibly a 30JJ - but I seem to remember her posting on Twitter that she was wearing a 32K Curvy Kate bra - because it was comfortable - which in US sizing would be a current 32O.