I missed too.
This is mostly because of the mod version, which means its my fault, not yours.

In 3.09 I rescaled the ASLs of all the submarines, and I took it a little bit too far. Acquisition range for this target, as you've said, is down to about 1000yd, and that's too small. You can expect a minimum acquisition range of about 1nm for the ADCAP in 3.10, which is large enough that you won't miss because it snakes the wrong way at the wrong time.
Not that it provides much more info, but this issue drew posts 40 and 41
here.
I recommend using a Kilo for target practice instead for the time being. It's not impossible to hit an Agosta by any means, but it's pretty easy to miss with an accurate shot.