Disclaimer: I am an SRE at Google, opinions are my own.
There's an excellent talk by Google VP of SRE Ben Treynor: https://www.youtube.com/watch?v=iF9NoqYBb4U. tl;dw: try to measure actual user experience, and make sure that even the long tile of customer still gets a good product experience. What "good product experience" means depends, on your product.
The rest of the error budget is for you to spend on releasing new features, changing the underlying architecture, etc.
There's an excellent talk by Google VP of SRE Ben Treynor: https://www.youtube.com/watch?v=iF9NoqYBb4U. tl;dw: try to measure actual user experience, and make sure that even the long tile of customer still gets a good product experience. What "good product experience" means depends, on your product.
The rest of the error budget is for you to spend on releasing new features, changing the underlying architecture, etc.