What would be the conversion if I used My 50/50 starter for the 1/2oz. fresh yeast?
That's a type of quesition that comes up fairly frequently but there is no really accurate way of converting commercial yeast quantities recited in dough recipes into natural sourdough (starter/preferment) quantities because each starter/preferment is unique in terms of its inherent “DNA”, the way it is refreshed, its hydration, and its degree of readiness.
What I usually do is to convert the basic dough formulation to a preferment version by determining how much preferment I want to use, either as a percent of total formula flour, total formula water or total dough weight (which usually is determined by the type of preferment I want to use—e.g., poolish, sponge, biga or prefermented dough), and calculate what remaining ingredients will go into the final mix. For natural preferments, I use the preferment dough calculating tool at http://www.pizzamaking.com/preferment_calculator.html
since it is set up to do the kinds of math involved. You will still have to do some number crunching with pencil and paper with a dough formulation that needs conversion, but the tool does most of the heavy lifting beyond that point. Not all dough formulations fit the tool. It is limited to those formulations that include a natural preferment (with possible supplementation with commercial yeast), flour, water, salt, sugar and oil. Not knowing your specific recipe, it is hard to say whether the tool will be of any help.
For bread dough recipes, it is not uncommon to see a preferment usage that is about 20-40% or more of the total formula flour. When in doubt, I usually start at around 20% preferment as a percent of total formula flour. I have done this on many occasions when converting commercial yeast pizza dough formulations into preferment versions. I had had less experience with bread dough because I spend most of my time with pizza doughs.
You might find this item of interest as you consider reformulation of your commercial yeast recipe to preferment usage: http://www.nyx.net/~dgreenw/howdoiconvertyeastbreadrec.html