I regularly get called upon to test the transmission of various filters with the spectroscope (R=10000) which has a resolution of close to 0.6A (at Ha)
The par focal requirements of narrowband filters is not related just to the thickness of the filter....
The formula for the shift along the optical axis when a Plane Parallel Plate (PPP) is used in air in a converging beam is well known and generally approximated to:
deltaZ = t -t/n2 for small angles where n2 the index of the plate.
For a constant filter thickness and base material, the only variable left is n2.
This unfortunately varies with wavelength.
Inherently then not all wavelengths can be par-focal unless ether the material of the filter changes or the thickness of the filter changes.
See p 40 of the attached:
https://wp.optics.arizona.edu/jcwyan...al_Testing.pdf
[Edit] Of course you're still left to handle any Chromatic aberrations in your system. I find using a Edmund Optics achromatic in the spectroscope the focus can vary up to 1.8mm between UV and Green and IR. In a 90/850 achromatic is can be 6mm difference!! This would never be able to be compensated by a narrow band filter.