This advice is kinda strange in my opinion .The easiest way for me to think of it is that 4% withdrawals are considered safe and you could do them forever. So take the income you want in retirement and multiply by 25. That’s what you’d ideally have saved. You probably need less considering you don’t live forever and you’ll likely collect social security or some other pension.
Have you tried rpm-ostree apply-live? https://coreos.github.io/rpm-ostree/apply-live/