If I want to double the amount of starter in my recipe, how much water should I reduce? Thank you.
It depends on the hydration of your starter. Your recipe should reduce the amount of flour by the weight of the flour in your starter, and should reduce your water by the total weight of the water in your starter. So, and example.
If the original recipe calls for 500 grams of flour and 500 grams of water, and 200 grams of starter at 100% hydration (50% flour by weight and 50% water by weight, or 100 grams of flour and 100 grams of water), and you want to double the amount of starter that you are going to use, you'll be adding starter that now contains 200 grams of flour and 200 grams of water. So reduce the water and flour in your recipe by the amount of extra flour and water being contributed by the extra starter. In other words, you will now use 400 grams of flour, 400 grams of water, and 400 grams of starter (which contributes a total of 200 grams of flour and 200 grams of water).
Note, do NOT make bread with the proportions here. This is only an example.
If you're interested in only reducing the water, you can figure the hydration level of the dough, and remove enough of the water to make the extra starter match that hydration. It sounds harder than it is. Here is my example recipe, which will actually work if you want to try it ;^)
100g starter @100% hydration
This recipe will make a 60% hydration dough. 50g each of flour and water are in the starter. So it would be calculated this way, (250 + 50) / (450 + 50) = 0.60 or 60%. To double the starter and keep it at 60% hydration, you would have to figure out how much water in the starter is excess. That's easy in this case. 50g flour hydrated at 60% would take 30g water. The starter would have 50g water, making 20g excess, so that is how much you would reduce the recipe by. The new recipe would look like this:
450g flour (We're not changing this, because we only want to reduce the water)
230g water (We only took out the 20g excess)
200g starter (simply doubled)
10g salt (Well, the recipe did have 2% salt before, so you could increase it to 11g to keep it 2%, but bread is very forgiving, so you probably won't miss it much)
Oh yeah, salt isn't the only thing that may be out of proportion in your recipe if you use this method of adding starter. Since everything relates to the flour, just reducing the water isn't really the most practical way. The way DivingDancer showed is actually the better way, because it doesn't end up adding anything, so none of the ratios change.
I actually did laugh ;-)