Even if hooked up backwards the reading is good. The meter is at 0, you stick your tube in and read it. If it moved .04 in either direction its good, because if it wasn't, the house would have already been full of smoke. Draft is the difference between the air pressure in the room and the air pressure in the pipe. The room will be at higher pressure and the pipe at a lower one. This is what causes the constant flow of air up the chimney. If there was no draft, you would not have a fire going at all. The difference between the two pressures is important with coal as it is a grumpy fuel. Gas, oil, even wood can be throttled over a wide range. Mr. Coal is only happy in a narrow band, too slow he zonks out. Push too hard and he takes out his anger on you.