Factor out niche value computation

[tautschnig]

Description of changes:

There will be future re-use of this code, so move it to a public function.

Resolved issues:

n/a

Related RFC:

n/a

Call-outs:

n/a

Testing:

• How is this change tested? Used in existing tests (in CI).

• Is this a refactor change? Yes.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[tautschnig]

Closing in favour of #2369, which is backed by a branch in my own fork.