Well, first of all, don't panic!
Second, your configuration with the
geometry package seems about right, so there shouldn't be any problems if that is indeed what you want your margins to be. You can see this by also setting the option
Furthermore, you can check your layout lengths with the
layouts package (see here)
Third, a stated by @samcarter and @Arne, there is a tolerance related to every printer and even every sheet of paper! So don't be so hard on yourself if you see some minor discrepancies.