If I was to replace I cup of flour in a recipe with 1 cup of sourdough how would you calculate the dough hydration? I believe my starter is what you call 100%. When I build to get enough starter for a recipe I use 1:1:1. One part starter, one part water and one part flour by weigh.
at the end of the day, dough hydration is based on the ration of water to flour by weight. you would need to adjust your recipe accordingly for weight.
1 cup of 100% starter is about 240 grams, and that can vary depending on the activity of your yeast (more active, more bubbles, more air, less dense, less weight, and converse is true). 1 cup of flour is about 120 grams. so it's not a one for one volume substitution.
If your starter is 100% hydration and you used 100g of starter and added to that 100g of water and 100g of flour, 1:1:1, your recipe would now have an additional 150g of flour and 150g of water. Add those numbers to the totals of flour and water in your recipe and calculate your hydration from there. If you're basing your percentage of hydration in your starter on volume instead of weight it's going to be hard to figure with precision.