One of the most important things to think about when making bread is what hydration level you are using in any given formula. Hydration is defined as the total amount of water divided by the total amount of flour.
So to start off, you should figure out how hydrated your starter is. One of the more common feeding schedules for starters is 2:1:1 (starter:flour:water). This means that if your starter weighs 100g, you would feed it 50g flour, and 50g water. Since it is equal parts flour and water, your 200g starter would now consist of 100g total flour, and 100g total water. 100/100 would put you at 100% hydration.
Starters can be kept at many different hydration levels and still be active. Some people keep their starters as low as 50% hydrated, which would mean that for every 100g of flour they feed their starter, only 50g of water go in. On the other end of the spectrum, you could be at 125% hydration, which would mean that for every 100g of flour, 125g of water go in.
It is ok to keep your starter however hydrated you would like, but keep in mind that there could be some differences in terms of how you use them. Lower hydrated starters (50-70%) are rather firm, and might need to be cut into many pieces to incorporate them into bread dough. Higher hydrated starters (100-125%) are very watery, and allow the starter to eat through its food supply (the flour) much quicker. For this reason you might need to feed it a larger quantity of food, or more often. I am beginning to like the higher hydrated starters, as they are much easier to mix, which allows for quicker feeding.
Once you know the hydration level of your starter, you can calculate the hydration level for the dough you are making. If you are using 300g 100% hydrated starter, 300g flour, and 200g water, your total flour weight would be 450g and your total water weight would be 350g. Therefore your dough would have a hydration level of 350/450 or 78%. If you had instead used 300g 60% hydrated starter (187.5g flour, 112.5g water), you would end up with 64% hydrated dough (312.5g water/487.5g flour). If you are adding other types of ingredients, remember to calculate how much water is coming from them; eggs are about 75% water, while milk is around 82%.
The hydration level of the dough will give you a lot of information about how it is going to act. Bagels are one of the least hydrated doughs (50-60%), and are extremely stiff. This means that they need a ton of kneading to get all the flour incorporated and gluten developed; it also means that they are not sticky at all in dough form.
As you move up in hydration, the dough starts to be a bit more sticky, but also more extensible. Many formulas (french bread, white sandwich bread, challah) use around 60-70% hydration. These doughs can hold their shape well, but also allow for a greater volume in proofing.
On the higher end of the spectrum you have breads like focaccia and ciabatta, which could be 75-85% hydrated. These doughs are extremely sticky, and need different types of handling. Because they stick everywhere, kneading does not usually work on these doughs, and instead you can use techniques such as stretch and folding, french folding, or just letting the dough develop the gluten over a long period of time on its own. These doughs can be roughly shaped, but because they are so wet, need careful handling. Because they are so hydrated, they might need a bit more bake time than usual prevent the inside from being gummy.
Recently I have been experimenting with having even higher hydrated doughs. One of my formulas, Baked Blarf, uses pretty much just 100% hydrated starter that has been dusted heavily in flour, and stretched and folded once. This dough does not hold together much of a shape, and needs to be baked much longer than usual, but can still taste delicious.
One final consideration is that different types of flours absorb water differently. Don’t be surprised if the consistency you are used to with one flour at 75% needs to be adjusted to 73% or 77% when using a different brand or type.