How do you work out how much sourdough starter to replace yeast

Hello,

Not sure if I have put this in the right place, my apologies in advance. 

I was wondering if there was a technique or perhaps some math for replacing yeast in a recipe with sourdough starter?

I recall a baker once telling me that fresh cake yeast is commonly used in a bakery but not in a home kitchen. So in place if you were to use a recipe from a bakery and use active dry yeast, multiply it by 0.5. If you have instant dry yeast, multiply it by 0.3. They never mentioned the math for a starter though. 

I love the long ferment of my starter and the flavour it gives but I haven't worked out how to adapt a recipe that calls for yeast and replace it with my starter. Then further more adjust the timing of things such as the bulk.

How have others approached this?

Thank you