I've been thinking a lot about the merits of the (highest income / median income) ratio in creating a society that is a little more resistant to eventual inequality. Rather than setting absolute tax brackets, what if we define tax brackets by this ratio? That way, it will be in the interest of the highest income earners to raise the median income.
That metric is very sensitive to outliers. If you have even one individual with enormous income, that ratio will be enormous. Now you might say "that's precisely the point", but I would argue that having how we design a society hinge on what one individual earns, is not statistically sound.
A more appropriate metric along the same line of thought would be something like top 10-percent excluding outliers divided by median income, where "excluding outliers" could be calculated in an admitedly somewhat arbitrary, but common way like e.g. https://en.wikipedia.org/wiki/Interquartile_range#Outliers.
Or just use something that already exists instead of re-inventing the wheel https://en.wikipedia.org/wiki/Gini_index