Lines dry tank still full

Hi, I have been trying to run my tank out of oil in prep for a new gas boiler. I thought I had successfully completed my mission just in time for April, but it turns out the lines are dry from the tank, both the return and supply and the tank still has 17" of fuel in it. What gives? I was told by a relative in the business that the pick up inside the tank is about 3-4" from the bottom. I blew both lines out from the heater with compressed air and they seem to be free of obstructions. This happened overnight which leads me to believe the burner is starved of fuel. I have left both valves open and in a bucket to see if the oil resumes it's flow. Does anyone know how far down the pickup is in a 550 gallon tank?