I have been summoned.

It can't be done. No matter what metric you use to track, it will be to some degree inaccurate and will be gameable. Especially if you want a decentralized currency, suppose you say "the supply increase must track the price of table salt" well now any firm that can control salt production can manipulate the money.

This is why we make these decisions about how money supply should work and say "as long as everyone knows how this works it won't shock the market." There are 5 primary ways to manage supply. Hard cap, linear emission, geometric emission (percentage supply increase per period), self regulating increase and the government decides. The last one is obviously bad, the others each have their pros and cons. Self regulating increase, where increase in purchasing power leads to people creating new co,ns, is the best. Gold has this, if the value of gold goes up people mine more gold. We don't have a way to do that with cryptocurrency, you need an oracle for purchasing power, which is a central point of failure. There could be some schemes, something like Monero's adaptive block size but more like an adaptive block reward, I haven't thought on this or come up with any schemes and haven't heard any. Seems pretty complex and probably not accurate and gameable.