To my opinion, the name is not Lengendorf but Langendorf.
There is only one guy with this name in the saxon Verlustlisten in a artillery unit, Alfred Langendorf who was in the 5. (F) Batterie of the saxon Feld-Artillerie-Regiment 245 formed on the 28.3.15 near Rozoy (Aisne) from the former III/ Reserve FAR23 and the III/ Reserve FAR24.
des.genealogy.net
des.genealogy.net
I am wondering for what the "F" could stay for

Maybe Fuss-Batterie? One can see on the second publication of the Verlustlisten, that three Batterien of the nine have the "F" before. The fourth, the fifth and the sixth... If the marking 12 in your helmet really could have been for the FAR 12, the only link between FAR245 and FAR12 is the fact that part of the Reserve FAR23 (the first Abteilung) was formed in Dresden from the FAR12.
Why does this helmet has a spike instead of a ball? This could be a postwar change made by an inexperienced collector...
Philippe