For sourdough recipes, I go with half a cup of liquid per cup of starter. Then add a cup an a half of water for two loaves. You can go more or less depending on how big you want the loaves or rounds to be. This fits in my standard size loaf pan.So, with this logic, 1 cup of water + unknown amount of flour + how much starter? And I assume this is for a starter only recipe or is this a starter + extra yeast?
I like it because you can easily shrink and grow a recipe without doing too much work. Baking is a bit of chemistry so some things are important. other things not so much