I think there is very little point working out a formula to keep you just out of visual range as you move to your intercept point. This is mainly because your victim's visual range is always going to be changing due to several factors (time of day, weather, sea state, your profile and height and more). For example, if a fog bank sweeps across their visual range will drop considerably and you will be able to take a short cut to an intercept point. Conversely, if your are heading to an intercept as dawn approaches, you are going to have to steer around their ever-increasing visual range.
If you just want the most efficient intercept course, just make sure your victim remains on the same visual/hydrophone bearing as you approach. This is good when you are aproaching from way outside their visual range.
You piqued my interest with your diagram, plotting an intercept course from an initial parallel course (just outside the victim's visual range), so I plotted it out myself (in 30 minute iterations). Guess what?... it turns out to be an ellipse (s'pose I should have seen that coming). Trouble is, an ellipse will be a bu**er to plot!
Still, all a bit academic, when visual ranges wobble all over the place!
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