It doesn't have to be that complicated. Just absolute terms could have the desired effect.
- 1% tax of 10 million = 100k
- 20% tax of 100k = 20k
Hence, the rich pays more tax.
- 1% tax of 10 million = 100k
- 20% tax of 100k = 20k
Hence, the rich pays more tax.
If there was an unequal vote, I am pretty sure rich people would want votes as heavy as possible.