Since it seems your using auto target, green is the best angle. But really the best indicator is the TDO.
Its a slide out tab on the uper left of the periscope screen. There are two pages. One sets depth and speed, the other is for setting AOB and apparent bearing. With auto targeting the latter (the PK) is set for you automagically, however you can watch it to see how big an angle the torp would have to veer to hit its target.
Anything within 15 degrees to port or starbort off you bow or stern (depending which tubes are selected) you should be fine.
Here is a pick its the dial at the bottom in this case the target is about 5 degrees off to port an acceptable angle.
