I'm just starting to delve into the whole prefermentaion thing, and I'm very interested in creating my own starter from the yeast wafting around my house. One thing that's been bugging me is this:

What I've been reading suggests that, to feed your starter, you basically take away a certain percentage of it, then replace what you took with a certain percentage of water and flour. Let's say, for example, 50/50.

However, how does this work given that some of the water that goes into your starter will evaporate between feedings (where obviously the flour won't)?

So let's say you start out with 100g flour and 100g water. Between feedings, evaporation has lowered the amount to 100g flour and 50g water (a bit extreme, but just for example).

So you take out half, leaving you with 50g flour and 25g water. You then add in your 50/50 mixture and end up with 100g flour and 75g water.

On the next feeding, another 50% of the water is gone, leaving you with 18.75g of water when you discard and 68.75g when you refill.

Won't the start eventually dry out?

When doing the final recipe, how do you figure out how much water is in your sponge for the purposes of figuring out how much flour to use?