Found this website that lists down the proportion that each member gets for the various Berryz songs. Red indicates highest number, while blue indicates lowest.
Not sure what the exact measure is, but I think it's number of beats for each member, and any shared lines get split amongst them.
I'm surprised that Miya seems to get the most lines in singles. I would've thought that Risako would've got the honour.
I wonder if it takes into account all of Miya's 'oohs' and 'ahs'
There are also one for the other H!P groups, but nothing surprising there