I have come across a recipe that I would like to use my starter in instead of IY. How would I go about doing this---is there a formula?
Keep the same total flour and water and Pre-Ferment a percentage of that. How much you choose to Pre-Ferment is up to you and the results you wish for. There is no one formula for substituting sourdough starter.
You may be substituting sourdough starter in place of IDY but whatever you end up with will be a different bread. One is sourdough, the other isn't.
Things to take into account is flavour and how much time you have to devote to it.
I usually look at what % preferment I use in my SD formulas and then follow suit. I tend to use a rather large percent for my levain around 25-28% as this works best for most of my formulas that I like to bulk ferment overnight.
I agree with the others that there is no one 'formula'. However, there are several things you can try (basically starting points). Take your regular recipe and discard the IDY. Then take about 10% or so of the total weight of flour in the recipe (large variation here, depending on the type of flour and the kind of bread you like, hydration level, etc. but start with 10%). If there are whole grain flours in the mix then use at least some of them in the levain (starter) mix. Add the same weight of water (e.g. if you are fermenting 100 grams of flour, then mix in 100 grams of water) and a spoonful or so of your active starter. Let this sit for most of the day until it is bubbly, puffy and floats in water. Mix the dough when the starter is ready.
Here's where the experimenting comes in. This will likely take a lot longer to rise than the same dough made with IDY, so give yourself time to watch it develop. I like to mix sourdoughs in late afternoon / early evening so I can do stretch and folds and watch for signs of fermentation. If it is very slow (usually with a small amount of levain this is the case) I'll let it sit in the cool basement overnight before shaping and final proofing. Otherwise, if it shows good signs of fermentation (rising, bubbles, etc) I'll pop it in the fridge overnight.
Sorry if this is too much information. It's difficult to know from your question what your current level of experience is with sourdoughs!