Dead reckoning is done every day, every hour, every minute in RL. Doing one or at most two fixes a day would be a lot. A "noon sun" is easy, but most of the time subs were patrolling at PD at noon.
Except in AE. They're always on the surface there.
In game this is kind of DR is assumed, what you get is a fix entered into the map at 2 hours intervals or anytime you ask for it. It also take only a minute or so, game time, while the celestial fix takes around 40 minutes, game time, and it alsways done at noon, midnight and 6am
Game also have an optional enigma machine, so a few important messages come in code, and you would need to use enigma to decode it.
Definitively recommended for any submarine simulation gamer; this is like an evolved SHIII-GWX mod
Important to mention, you need the non-steam version of the game to install the mod
I played SH4 a fair bit years ago. Despite my much greater interest in the PTO and the USN sub war, I could never get past some of the design decisions they made. Always went back to SH3 Grey Wolves.
The sight reduction math for a fix takes some time and a bunch of books. Six hours would be OK. Weather decides when you can get a good set of shots. You have to be able to see a horizon to use a sextant, and that's often more limiting than the stars and overhead clarity.
From memory, but I think Enigma was purely Germans, The intercepts in the PTO were Magic. I think one other system too, but the name escapes me.