replacing instant yeast with SD

as to what you are trying to do.  A 1,000g loaf of bread wet, can be risen with 1 g of levain, 50 g of water and flour made into a levain and allowed to ferment on the counter for 24 hours.  Or you could use 20 g of starter and 50 g each of flour and water for the same bread but it takes less time to build the levain say 6 hours in the summer.  OR you could use 100 -200 g of starter and forget the levain stage all together.

The thing to remember as a rule of thumb, is that you need between 10-20% of the final weight of flour and water of the loaf to be a levain at full strength to get s nice loaf of bread in some kind of reasonable time.  Just take some of the flour and water from the recipe at the hydration of the final dough and make a SD levain out of 15% of the flour and water in the recipe.

If you want more sour go longer with a smaller amount of seed starter and lower percentage of levain.  It will take longer no make bread no matter hat you do but it sure will taste better.,

Happy baking

This is the formula that I've been following for the last four years.  290 grams active starter, 880 grams flour, 2 TBS salt (optional), 540-550 grams water.

My routine is to mix the sd, flour, water the night before.  I like to knead because I like to feel its transformation into bread.  I can also determine if it needs more flour or water (depending on the weather). I've been doing this a long time and know how it is supposed to feel.  I knead for 10 minutes or so, cover and place in refrigerator. Next morning I form into loaves.  ( Free-form loaves are too difficult to slice for me so I divide the dough into two loaves of equal weight.)  Before I form the loaves I add a 1 ounce mixture of flax, bran, and oatbran to each.  I knead it it and form it and place into well-oiled (olive oil) cast iron loafpan to rise covered in plastic wrap.  The bran mixture keeps the plastic wrap from sticking.  It rises for 7-8 hours.  Then I bake in a 420F oven for 40-45 minutes.  I used to preheat the oven but I find the bread turns out identically either way and saves on fuel.