Easiest way in dire situations:
1.- Point your bow to the target (You can now also make a quick fixed line speed measurement before going to the next step)
2.- Estimate AOB. Say for example you calculate it to be 30º
2.- Quick maths: 90º-30º=60º
3.- Now turn your bow those 60º in the same direction the target is going.
You are now at right angles to the target course