I apologise in advance if this subject has been covered elsewhere.
This week I have been following the recipe for sourdough culture from the Fabulous Baker Brothers cook book but I do not understand the maths. To start the culture they recommend 75g flour + 75g water. Repeat everyday for a week. (5 x 150 = 750g). The recipe for a sourdough loaf requires 300g of culture (including 150g flour/water from the day before). So far so good. The plan for feeding the culture states that you start with 75g of sourdough, feed with 75g flour and 75g water (225g). Add 150g flour/water the day before you plan your next bake (375g) thus leaving 75g of culture to start the whole process over again.
After baking my first loaf I am left with 600g of culture when I only need 75g so why was it necessary to start with 750g in the first place? Is it better from a quality point of view to keep a large reserve or a small one?