[magic452]

To shoot a "Fast 90" you must do two things.

First plot the target's course.

Second plot your intercept course so that when you get to your firing position
you are perpendicular or 90° to the target course. Hence Fast 90.

If the target is coming from your right you will be shooting at the target's left or port side. Coming from the left, shooting at right or starboard side.
Set the AoB port or starboard depending on which side you'll shoot at.

You set AoB to 90 because that is what the AoB will be when you shoot.
Might not be exactly 90° but very close.

I don't know what the "video from YT" is but make sure you are not mixing up two different shooting methods. If he is setting the AoB and updating it, it sounds like some other method.

Magic