976: make under/overprediction calculation concise

[sbfnk]

Description

This PR closes #976.

As mentioned in the issue I used the opportunity to make under/overprediction calculation concise.

Checklist

• [X] My PR is based on a package issue and I have explicitly linked it.
• [X] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [X] I have tested my changes locally.
• [X] I have added or updated unit tests where necessary.
• [X] I have updated the documentation if required.
• [X] I have built the package locally and run rebuilt docs using roxygen2.
• [X] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [X] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.