You don't need an objective measure. An approximate one is already better than just "optimising for human happiness". And I guess you do what I proposed alread

You don't need an objective measure. An approximate one is already better than just "optimising for human happiness". And I guess you do what I proposed already anyways: The suffering of a dog or a horse is probably also - to some extent - your concern. The suffering of a piece of rock probably less so. Should we learn that rocks can feel pain, too (granted this is unlikely), we should update our morality. It's also part of most countries' laws to prevent unnecessary suffering of animals. And this is all based on the assumed degree to which they are able to suffer. You will get fined for torturing a dog; you won't for torturing a mosquito. Careful - fancy thoughts: I could imagine that we will update our morality regarding plants one day (given that we see more and more evidence that they communicate with each other, send stress signals etc.).