Your multiplier is too large. Don't use 10x, like you're using in your example (i.e. you're going to go 10 knots, multiplied by 10, so you make a 100km radius circle). Use something like 3x or 5x.
If you used 3x, for example, that means that you would only measure 18 km down the target ship's vector, instead of 60 km as you've done in your example. Then, you would make the radius of your circle 30 km, instead of 100 km as you've done.
Give that a whirl and see what you come up with.
