A method and apparatus for determining the longitudinal position of a tapered displaceable element positioned between two substantially orthogonally laterally opposing displacement sensors. A change in the longitudinal position of the displaceable element causes the sensors to each measure their distance to the displaceable element which relates directly to the local thickness and thus the longitudinal position of the displaceable element. The system factors out errors in measured lateral proximity position of the displaceable element since an erroneous proximity to one sensor is equal and opposite to an erroneous proximity to the other.