I am attempting to determine how strong a magnetic field will be on the other side of a thick material and then across an air gap. For instance, I have a closed steel cylinder with a steel piston inside it. I plan to affix a permanent magnet to one side of the piston (inside the cylinder). I am looking to use another cylindrical magnet that can slide inside a separate stainless steel cylinder mounted outside and parallel to the first. My goal is to gage the position of the steel piston within the steel cylinder by viewing the position of the second magnet through windows in the SS cylinder. It will be necessary to have a small air gap between the two cylinders for mounting purposes. Hopefully this description makes sense. A hall effect sensor would work great to determine the magnet's position but I need a mechanical analogue thus not requiring any electric signal. Precision is not terribly critical, with relative position being more important. So, I am attempting to calculate the required strength/size/material of the two magnets based on the thickness of the first steel cylinder and the air gap between the two cylinders. Thanks!!