I am aware of two calculations for yeast starters. There is the Chris White/Jamil Zainasheff model and the Kai Troester models (https://braukaiser.com).
The White/Zainasheff model is based on a diminishing return where adding more starter wort ultimately gives you no more yeast growth.
I've been using Brewersfriend for everything (recipes, water calcs, yeast calcs). Their yeast calculator uses the Braukaiser model for the first step, and the Chris White model for the subsequent steps. In their discussion about the calculations, they note Braukaiser should only be used with stir plates.