The only thing about this is that EP FL and FR of telescope is not an exact science.
It is close enough for practical purpose but let's go with some numbers. My 102mm f/7 APO's have a 714mm focal length. Let's be generous and say that the focal length of the scope could vary by as much as 25mm. And lets say we are using a 10mm eyepiece that could have sample variation as much as 0.5mm on the focal length (so 9.5mm to 10.5mm).
So using the published 10mm and f/7 gives an exit pupil of 1.43mm.
If we adopt a 25mm uncertainty in the scope focal length then the scope focal ratio could vary from 6.75 to 7.25 which means with a 10mm eyepiece the exit pupil would be 1.38 to 1.48mm
If we now also add in a hypothetical 0.5mm variation in the actual eyepiece focal length to the 25mm uncertainty in the telescope focal length then the exit pupil could vary from 1.31mm to 1.56mm. So in my extreme example the deviation of the actual exit pupil from the value provided by eyepiece focal length/telescope focal ratio is only +/- 0.13mm. And real optics are almost certainly more accurate than that.
I just don't think we should confuse people trying to understand their optics by suggesting the calculation is not reliable - since it is reliable to any reasonable backyard usage.
Edit to add: Now there are some examples where rebranders mess with the eyepiece focal length to make it seem like it is a different eyepiece. The most extreme example I have seen is an eyepiece labeled 7mm that was actually an 8mm. Then you have one company for example that labeled their version of eyepieces with a 0.1mm difference from what the eyepiece is when branded by other companies. Or my 4.7mm APM UFF Ethos clone is labeled 5mm. These are small, inconsequential differences.
Edited by russell23, 26 March 2020 - 09:02 AM.