Average approval is only useful for elections. Minimum approval is what is helpful for population growth purposes.
I can't imagine a target income level being useful. We care about income, but only in the sense that it's at the maximum with respect to our target approval, so target approval should be sufficient.
Alas, this is probably too much to expect to be added to GC2 at this late stage, and even if it were added, the likelihood is it would only be added to the most recent expansion, Twilight of the Arnor, leaving Dread Lords and Dark Avatar players without it.
You can't autoadjust in the game, but you can at least open the Colony List and sort by minimum approval, and if you're decently familiar with the morale math, you ought to be able to find your optimal tax rate without too much hassle. It's how I do it, anyway.