Baby bottle nipples Qs

Ka

Hi! So I have a bunch of nipples from my daughter and I'm sorting them out and trying to figure out the flow/size of them.. dr brown bottles says level 1 or 2 whatever level the may be. but the others just have like s or m or f so I take it they stand for slow flow, medium flow or fast flow? Would anyone agree? I know its trial and error but I'd like to continue to stock up and this is has me stumped!