Noob AOB question

HI I'm pretty new to this but have mastered most of the 100% realism charting and navigation stuff however I have a couple of questions, which are probably really stupid but i just don't understand.

1) If I have set up a 90% perc approach to my targets track. The target is approaching from my Port(Left) - Why do I set my scope at O degrees and then set the AOB to STARBOARD? ? ? Surely if the target is going to be at 0 degrees when I fire neither port or starboard.

2) What the best way to read a targets bearing without watch officer etc - Currently I just take my "scope bearing to target" and add my current bearing (Minus 360 if the answer is above 360) ...Is this correct? Is there a better way?

Cheers guys - Great forum / community here