I'm a little skeptical about this procedure. If I understand it right, you have shifted the TUM grid to align with MN, which is fine if you want to be aware of MN variation and dispense with all that math. This is certainly advantageous for a lot of uses.

However, you have just disoriented the UTM grid and any coordinates you have or might obtain are useless. This is very bad for other purposes. How would you give your location to SAR groups? How would you fix the position of an archaeological site (my particular usage)?....