So you are plotting by that big rotating bearing wheel centered on your sub? I never really liked that. 1, back in the day my pc performance suffered from it's large size. And 2, it moves while you are trying to get the plot down. So it is hard to get the bearing or range on the proper grid position.
Instead I make, or I should say made as I barely play SH3 thesedays, frequent use of some GUI tools and a physical DIY attack-disk, or handheld calculator.
At the time to plot I ask the watchofficer for a range and (relative) bearing.
Immediately I plot the start of a line where my Uboat is then. I leave the end of it in the rough vicinity of where the target could be.
To plot lines in a specific direction you need the true bearing (or heading) to the target. You can calculate it as owncourse + relative bearing. Then subtract 360 degrees if you need to get the result below 360.
My physical attackdisk is used by rotating the bearing backplate to the compas scale for my course at the triangle representing own course. Then turning the bearing pointer to the relative bearing, and reading the true bearing on the compass disk. This is quickly done fractions of a second.
Now, knowing the true bearing to target and range to target I can update the end of the plot line using the bearing wheel on the mouse cursor.
I made my own small mod expansion based on the OLC gui mod I had at the time. This adds the compass scales to all line tools on the map. Although GWX3 already has such a tool for the line iirc. Anyway, this tool won't disappear from the map when zoomed in. And you can take your time because you are no longer chasing a moving grid. Much better accuracy of the plot.
As I recall the default Watchofficer reported range in units of 100m. This is precise enough. Perhaps even too much. That was the reason they added some randomness to it through the HSie/Stiebler patch SH3 executable mod. If you have that applied that may also be part of the reason your plots may not be matching to the visual perceptions of AOB.