15×15=225 and 15×15=225 225+225=450 and the square root of 450 is about 21.2 but that would be a straight line. Pulling the line through the water will cause lift on the line and is going to create a bow in the line so depending on the size of the line will determine how much bow is in the line causing you to have more line out. So 21.2 is not the correct answer.
You will need to find someplace that has 15 FOW and drag a bouncer with a line counter reel at the desired speed to target the suspended fish.