Intelligence, strength, adaptability, etc. Catch 22. You can predict these things, albeit not perfectly, by looking at the indicators for success. Socioeconomic standing of the family, genetic history, etc. However, I'd argue that in even attempting to do this, you're opening up to a lot of corruption. Ergo, I'd prefer to just skip it and leave the minimums where they are instead of trying to even figure out what the criteria would be.
That's kinda the point. It's not possible until it's too late (meaning until the person already exists and you have to decide they need to die since they don't meet the minimum standard). I'd prefer to let society freely decide who gets what resources on it's own instead of trying to force my views (or worse, someone else forcing their views on me because they get the governmental power) via government.
In the same way it makes no sense to try and discern, objectively, what makes a person "worth it" to a society, it makes no sense to try and limit what a society thinks is "minimum standard quality of life." In reality, there is no such thing as minimum quality of human, and therefor there should be no minimum quality of life.