Just a "techie" here (god I hate that term, but anyway...)
How about union membership as a % of your income. That's how it's done with AIPA. 0.75% from memory. Haven't heard anyone EVER complain about it, and let's face it, our salaries vary a lot more than yours from the bottom to the top (i.e. A330 S/O vs. B744 captain is a difference of over $250,000).
Just a thought...