Silverleaf, I think you took a wrong turn at step 7
When you draw the circle, look at the point where it intersects a line connecting your position and the target (the protractor leg you made in step 3). Pick up the protractor, draw a line from the center of circle to that point I mentioned above, and to the target. Note the angle of the protractor.
Take the protractor again, with that same angle, from the target into your boat, towards the target's path. Where that second leg intersects target's path is th spot you will intercept it. That second leg is your course. Use the bearing tool to measure it, or make a waypoint.
But if you follow that course at the speed you used to calculate the intercept point, you will reach it at the same time as your target. Calculate your speed to reach the point about half an hour before the target to prepare your attack.
Also, for "slow" targets use 7kts, not 6, as this is the maximum speed they could go. For "medium" it's 12kts.
Hope this makes sense. If not, ask.