[Pisces]

[EDITED for simplicty]
I take the TARGET COURSE.
I subtract the TARGET BEARING from it. (don't care if the result is negative or beyond 360)
I take the sine function of it with a scientific calculator. (make sure it is in degree mode, not radians or gradians)
I multiply the result by the target speed.
I divide by my chosen intercept speed.

If the result is less than -1, or bigger than 1, then forget about chasing it, or speed up and redo the calculation. Bigger than 1 (or less than -1) means he's outrunning you. Exactly (-) 1 means the best you can do is move perpendicular to the bearing, and let him close the distance to you.

Take the inv(erse) sin, or arcsin, or asin, or sin -1 (!superscript) of the result.

[EDITED] Add the arcsin result to the TARGET BEARING. If the asin result is negative, then adding a negative is the same as subtracting from the TARGET BEARING while ignoring the sign.

Head that way, unless it keeps him in your sonar deaf-zones. Then you have to turn a bit to keep an ear on him, and make do with a bit less efficient intercept.