Contact
Bug reports, wrong answers and suggestions all go to the same place.
Write to support@solvecalc.net. It is the same inbox that handles the main site, so mentioning that you are writing about the GitHub Pages build gets your message to the right place faster.
Reporting a Wrong Result
Three details make a report actionable: the exact expression you entered, what the screen showed, and what you expected instead. Add the browser you were using and a problem can usually be reproduced immediately. Without them it rarely can.
Issues and Pull Requests
Because this build is open source, a GitHub issue is often faster than an email for anything code related. Open one at github.com/solvecalc/solvecalc.github.io, and include the page URL.
Suggesting a Tool
Tell me the calculation you keep doing by hand and which year group it belongs to. Requests that arrive more than once tend to get built first.