I ended up solving this by including this line in the _redirects file in the base domain (www.[fullname].com) repository, instead of in the project repository.
/Sourdough-Calculator/* https://sourdough.[fullname].com/:splat 200
I figured it out after reading this thread: How to redirect? - #6 by fool
For the future, I think the documentation could be improved, or better yet, working with redirects could be in the domain settings on the Netlify dashboard.