converting commercial yeast bread recipes to sourdough

Have been an avid soudough baker for decades and would like to be able to convert a lot of good, standard method breads to sourdough.  Most of my attempts at converting  recipes have been hit or miss and I wonder if anyone has found a more scientific method of conversion.  For example, how much of a firm, levain type sourdough would equal 1 tsp dry yeast? Eventually, I always get a loaf of bread, but I'd be interested in something a little more consistent. Any thoughts?