Ah, but what is a quality lifestyle?
We confuse material wealth with a quality life. But the essence of a quality life start with TIME to LIVE as you choose. NOT with more stuff.
I propose that we don't start by determining the amount of de-growth. We should decide what is the minimum needed. That would be ONLY healthy food, medicine and the like. NOTHING should be produced for profit. Every bit of unnecessary production is a waste of time and life and additional toxins we'd be putting into our air, water and soil.
Once we value our time, and use it to enjoy each other's company, to enjoy Nature, to engage in creative works, and to wonder—THEN we will have quality lives.
That would be the sane approach.