Fix Shellcheck SC2125

[dcooper16]

This PR fixes Shellcheck issue SC2125 by enclosing arithmetic expressions in $((...)) in places where Shellcheck issues a warning.

What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change: bug fix, feature or improvement that would cause existing output (especially JSON, CSV) to not work as expected before
• Typo / spelling fix
• Documentation update
• Update of other files

If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md
• My code follows Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order of last name) and the change to CHANGELOG.md

AI section

• I found a bug / an improvement using LLM version: [e.g. GPT-A.B, Claude <NAME> A.B, Gemini A.B <NAME>, Qwen<B>-Coder, DeepSeek-<A> etc.]
• My contribution does not include any AI-generated content
• My contribution includes AI-generated content, as disclosed below:
  • AI Tools: [e.g. GitHub CoPilot, JetBrains Junie, VS Code plugin <NAME> etc.]
  • LLMs and versions: [e.g. GPT-A.B, Claude <NAME> A.B, Gemini A.B <NAME>, Qwen<B>-Coder, DeepSeek-<A> etc.]

[drwetter]

Thanks again