I am liking Ralph's approach but as Slayer states the curvature of the line through water resistance throws off the math a bit. As far as the downriggers, I found through use of my depth finder that with a 10 lb weight down "X" number of feet, moving at 3 knots and the lure 30 feet back off of the ball, with deflection in the downrigger cable, the lure was running at the depth the downrigger was set at. That was with a medium umbrella and two 2 or 3 oz bucktails on either side.
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