If a recipe calls for use of a FIRM starter can I use a more batter-like starter? If so, in what percentage? Same weight? or volume? or more or less? Thanks
A good starting point is to think of a firm starter as 50%. This means the weight of the water in the starter is half(50%) of the weight of the flour. So if you have 3 oz.(or grams etc) of firm starter, 2 oz is flour and 1 oz is water. So water is 1/3 of the firm starter by weight, and 2/3 flour by weight.
Likewise, your batter-like starter is likely 100%, meaning the weight of the water is 100% of the weight of the flour. So you have equal parts of water and flour, by weight. So 2 oz of starter is 1 oz flour and 1 oz water.
So you see, if you want to make that batter starter equal to the firm starter, you would simply add 1 more oz of flour. Of course you will want to let this ferment to maturity(3, 4 hours or so at room temp). Likewise, to continue building it as a firm starter, you will feed it 2 oz(or grams, etc) plus 1 oz of water(or 4 oz flour + 2 oz water, etc).
In short, divide the weight of your batter-like starter by 2, then add that amount of flour to it. I will seem to become very dry. Just knead it with clean hands.
If you don't have a scale to weigh your flour and water...well, I'll let someone else chime in that part.
Or just tell us how much firm starter(by weight) starter you need.
Northwestsourdough.com has a good tool that converts different hydration starters.
Just a reminder to add more water to the final dough to replace the water not used in your starter so your hydration stays the same. (ie If you use 100g less water in your starter simply add 100g extra water to your final dough.)