I found this today in a video by FoodGeek. A Bread Receipt calculator for figuring hydration, increasing for multiple loafs. Very Simple, Very Good.
The video gives a great tutorial.
Here is a link to the calculator itself.
Hey this is great! Thanks for sharing. Only slight headache is that if you make more levain than you use in your recipe, you have to guesstimate a little bit the actual amount of each ingredient in your preferment here. But once you've got that it's a wonderful tool for those of us who find baker's math to be confounding.