Aloha and welcome aboard!
It's good to have you here.
You didn't mention what kind of sextant
you purchased but I'll give you a reference that explains how to adjust your mirrors and scope
so you can sort it out.
See if your military library can get a copy of The American Practical Navigator
by Bowditch. It has a chapter on sextant adjustment that will help you a great deal. The other option might be to see through a search on the internet
for that particular book and see if you can get to the section 1610, 1611, 1612 which are pages 278, 279 and 280 of my copy.
I learned to sail on Chiemsee in Germany while I was stationed in Stuttgart but that's a very long drive. Also there is a small lake where sailing regattas are held in Stuttgart near what used to be called Robinson Barracks. I remember finding it while hiking and biking down the hill through the Garden Plots with my young daughter back in the early 70s. See if you can locate that lake on a map of Stuttgart (maybe a mapquest of google