It may be easier to rotate the domain in Fluent to align with y & z and then alter the gravity vector to suit. The thickness is then whichever of y or z is normal to the surface. Otherwise you'll need an expression in CFD Post to do the trigonometry to get the film thickness (function of y & z) against the wall position (function of y & z).
If you just rotate the domain the data moves with the mesh so you shouldn't need to do any additional iterations.
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