You're welcome to use whatever metrics you prefer, but in my opinion, it doesn't make much sense. When you hire someone, you expect them to do their job and compensate them accordingly. How they use their earnings is their decision. By asking people what they're using their own money for, you're stepping beyond the bounds.

Witnesses should ensure the reliability and security of the Hive network. That should be the main expectation.

I agree with @deepresearch, that people who hold significant amount of HP tend to be more credible.

Nonetheless, it's positive to see you engaging with the voting and doing research on whom to vote for, even if I disagree on the method you use.