I believe Hall sensors measure magnetic variation through voltage. In a DC motor (probably what your actuators are using), the voltage varies when the motor spins. Counting the variances gives you a precise measurement of the number of turns the motor makes.
So, if I'm right (ha!), your Fusion Brain should measure the voltage from a Hall sensor and increment a counter each time it varies. It should also know which direction the motor turns so it can increment or decrement the counter.
You can probably calculate that something like 100 turns = 1" or some ratio you want. You store those values in a location that you can retrieve and you can set the seat to the desired location.
Hall sensors can also be used as proximity sensors, so there may be some in the actuator that tell you if it is extended or retracted. There may be some way to determine from the readings exactly where each actuator is located, but that will take some figuring.