Thank you all for your comments!
@Dougie: Yes I will upgrade the D/F Loop, the insulating conduit is changed already. I have the Whaterline markings from Engel, but I can´t see some of this markings on pictures of U-404. So I dont know if I should use them.
I think Engel choosed 1:34 because of the technics. The maximum Pistontank Volume from Engel is 750ml, so the do not have to manufacture extra big Tanks and a bigger pressure Hull. Maybe that is the reason, I dont know.
@Helmut: I use a sheet of white decal paper. You have to stick two blades from a cutterknive (dont know the right term, maybe box cutter blade) together. So you can cut with two blades a very fine line. It only works if they are extrem sharp that the edge of the line is clean. I hope you understand, if not I will make you a few pics with every step.
best regards
Sascha