Bra intervention, how to calculate please

[Nanodust]

So I'm sure the bra I'm wearing just know isn't the correct fit. I've tried to follow the instructions for bra intervention and measure my rib cage at 29inches and at widest point when leaning at 90 degrees, 38 inches. How do I calculate what this is though?

I currently wear a 32D, but are my measurements a 30G?

Because you are an odd number you would want to try both a 28 and a 30 band. Try a 28G, 28GG, 30G and 30FF

Thank you!