I put together an Excel workbook for working with bread formulas. Although there are other similar tools on this site, this one has some nice additional features. Let's say you have a formula for a sourdough bread, but you want to make a couple changes. First, you want to add 10% spelt flour, you want to up the hydration from 65% to 68%, change the salt form 1.8 to 2%, reduce the dough yield from 3.5 pounds to 3.0 pounds, and increase the percent of pre-fermented flour from 15-20%.