You're using "income" as your input variable to calculate "fairness", but you could just as easily use "disposable income". Someone making $25,000 per year might have a disposable income of $0. Asking this person to kick in $5000 seems ludicrous.

I honestly don't know how you could assess fairness without looking at ability-to-pay:

(income) - (basic survival needs) = (money leftover you can potentially contribute to the government)

This is obvious, fair, and is why most nations have progressive taxation structures.