From volume to mass
I have for a year or so now been using a San Francisco sourdough culture I bought at the supermarket. It takes the form of a batter and works fine. I'd like to get more scientific but everything in the included recipe is in volume units, so for instance:
Mother sponge is fed with 1c flour to 1c water
Baby sponge is 1c mother, 1.5c flour, 1c water
Bread is 2c baby, 2/3c water, 4c flour, 2tsp salt
In the process this recipe seems to self-destroy information about mass. Is there any reliable way to figure how much water and flour is in that final dough? Each cup of sponge, mother and baby alike, is denser than water but also full of bubbles. Their textures are noticeably different but their densities aren't. Do I need to go back and start feeding the mother by mass instead of volume, both weigh and measure out every single step, and hope I get enough info that it works out eventually?
The recipe also suggests mixing the leftover baby back into the mother, which would really throw things off, and really doesn't sit right with all those chemistry classes I took where you never put leftovers back for fear of contamination. So I haven't.
Finally, most sourdough recipes I find don't have a mother and baby sponge. A portion of the mother just goes straight into the dough. Is this some regional variant? Is it appropriate for the ostensible San Francisco style? I do like the loose mother sponge, it's really easy to maintain. To feed, just dump in the flour and water, and shake. To measure out, just pour.