To plot your Own Ship vector, you need to figure length (or distance) of the vector. In this case the vectors are based on the time frame of 6 minutes.
Since your Own Ship will travel 12 nautical miles in 60 minnutes, it will travel 1/10 of that distance in 6 minutes.
1/10 of 12 is 1.2 so that is the length of the vector.