After pondering some more,
maillemaker, I have a couple of suggestions:
First of all, I never use the 3:15 method in SH5 (I'm familiar with it from the old SH3 days, I just don't use it anymore). Instead I clock the target at 5 min and use the handy-dandy conversion table. Which brings me to another question of yours that I overlooked. You're using the outstanding TDW NewUI, so to access both that conversion table AND the recog manual you've been looking for, just click on the "clipboard" icon in the upper right vertical menu and all of your draggable charts etc. will appear. You just need to click and drag them into view. The recog manual you're missing is on the left edge of the screen and only visible as a little gold clasp peeking out. Click and drag as needed. The distance to speed conversion table is one of the three charts tucked away in the upper left of your screen (it's the one called "Seite 2").
Anyway, when using the ruler to measure distance traveled, keep in mind that the ruler is not a precision instrument, it will round off. Thinking back, no matter how precisely I've measured my 5 minutes, I've never once hit an exact distance listed on the conversion table. What I do is "round off" to the closest one listed, and that has always gotten me the right speed so far. If you go by the strick 3:15 rule, by the other hand, you might be off by a hair.
Finally, seeing as how you're using the 90 AOB method (I do the same to the point where I'll hesitate a lot to attack if I can't use it), you have to pay really, really close attention to the gyro angle indicator after you've done the initial setup. It has to be exactly on zero for you to be on the right bearing with the periscope.
And finally finally (mheh, there's always something more with me
), this may or may not be an issue, but I had a problem hitting in the beginning too for no apparent reason. I don't know if this is what "fixed" it, but once I've initially locked a target and identified it, I hit Space to unlock it again (but leave the TDC on, I just click "never mind" to close the WEP dialog), then I turn the scope to a bearing of 0, turn on the TDC manual input with the red button, set speed and AOB=90, lock it in, and then turn it back until the gyro reads zero and wait. Initially, I tended to leave the TDC dialog and target lock on. I don't
know if this messed up my firing solutions, but it might be worth a try to not do so. (The dialog doesn't matter, it just takes up space, but the Space Bar "lock" might).