Here's a formula to calculate eyepiece field distortion (FD), given that the eyepieces field stop (FS) and focal length (FL) are reliably known and trusted:

FD = [(FS/FL * 57.296)/AFOV] - 1

To represent FD as a percentage, just multiply the result by 100 and take the absolute value (ABS) of the result.

For the example of the 24 mm Panoptic (FL = 24 mm, FS = 27 mm):

FD = [(27/24 * 57.296)/68] - 1
FD = -0.0521

The percentage of pincushion (plus any FL induced error, if present) is therefore:

ABS(100 * -0.0521) = 5.21%

And since this is pincushion distortion (FD is negative), we can conclude that the TFOV for this eyepiece will be 5.21% less than would be indicated by the old formula:

TFOV = AFOV/Mag

PS: For those who are confused by the term "distortion", the one thing that needs to be made apparent is that pincushion, barrel, and/or angular magnification distortions do not involve such things as sharpness, resolution, contrast, transmission, color purity, lateral color, ghosting, etc... so they are not generally what first comes to mind when one thinks of image "distortion".