Bra size

I’ve only had victoria secret bras which I wear a 34DD I have this other bra my grandma gave me and it’s a 38C I can fit it only if I adjust the straps very tight . So because I have to adjust the straps do I knock down the size of the number or letter (if that make sense) so for example by be adjusting the straps will I really be a 34C or 38B (those numbers are just examples ? Because I wanna order some bras from online websites but don’t know my actually bra size