Most landlords who haven't raised rent in years aren't being generous on purpose. They simply never put a number on what holding steady costs. Inflation does the work quietly: the rent stays the same on paper while the euros it brings in buy less each year. By the time it's noticed, the gap can be substantial.
The break-even rent is your current rent grown by German consumer price inflation since you last set it. Reaching it doesn't make you better off, it only restores the purchasing power the rent had when you agreed it. Anything below break-even is, in real terms, a discount you're giving without having decided to.
What you can charge is a separate question from what inflation has done. German law matters here. Under an Indexmietvertrag, the rent is contractually tied to the consumer price index, so the inflation figure is exactly what you can claim, with at least twelve months between steps. Under a standard lease, increases run toward the local comparative rent (Mietspiegel) and are capped by the Kappungsgrenze: 15% over three years in tight markets like Hamburg, 20% elsewhere. Inflation on its own is not a legal reason to raise a standard rent, so the tool shows both the real-terms gap and the legal ceiling.
None of this is legal advice. Rent law turns on the specifics of your contract and your local Mietspiegel. If you're unsure, a tenants' association, Haus & Grund, or a lawyer can confirm what applies to your tenancy.