[SH4]Needs advice playing in 100% realism

Hi there! I've recently come back to SH4 , my last play was like 7~8 years ago. Luckily, I could still recognize most of the game mechanics and It wasn't too difficult to get myself up to speed. That said, this would be my first time going for a full 100% realism with TMO+RSRD mod(Esp. without map update+ cam view, i learnt to use manual targeting back then).

Now, to my problem. I started my career at early war and I find myself having a very hard time plotting an accurate intercept course to a convoy without the aid of radar. During the day, I'd stay around 10k~15k yards(~5nm), using hydrophone to find out the range, but the readings are very inaccurate(+1-3000yards difference each ping) most likely due to them being to far out and too close together(mixed up sounds). At night, I could get closer(~8k yd), eyeball and guesstimate the course. I'd risk getting detected by dds using active sonar, even If I do, the readings would be,again, inaccurate and mixed up. Not to mention, they seem to change course once in a while, thus my math would be useless by the time I figured out their general direction.

The large convoy(12cargo+3 DDs) I'm facing right now is even more troublesome, they'd literally start shooting at me while i'm ~10k yd out in daylight. I have no idea what to do since I can't plot their exact course, tho I know that they're located east of Singapore and is headed toward east of Sumatra.

I have no problem plotting and intercepting a single or a few ships, but when it comes down to a decent sized convoy, I'm clueless.

I've spent 4 hours trying different approaches with no avail, Any advice?