https://en.wikipedia.org/wiki/Pensions_in_Germany
I think you want to have all three pillars going, normally you save in some kind of stock market pension fund.
I don’t live in Germany, but lot of countries in EU have similar system. Over here in Sweden, you can easily see an overview and your projected pension at retirement age by logging into one of government offered e-services, maybe Germany has something similar.
What number you need to reach - is completely up to you of course.



You can use Calibre + DeDRM and some jiggery pokery with Windows version of Kindle, but to be honest, that’s just really fucking annoying and it’s just easier to sail the high seas and get already open version of your book.