Well, that's NF's problem. But just for fun, I went to their website and found information about the ATACR 5-5X56. The FOV at 5X is a small cozy 18ft @ 100 yards. On the other hand the FOV at 25X is respectable 4.9ft at 100 yards. So if we flip the formula around and use the FOV at max instead of min magnification, the FOV for that scope at 18X is 6.8ft @ 100 yards, as displayed in the list earlier on.I think ILya covered this with his video on apparent field of view, unfortunately it is not a linear formula but I agree this probably gets you close. A good example of a scope that doesn't fit the formula well is many NF ATACR scopes, they have fairly pathetic FOV numbers at the bottom of their magnification due to their design, but as you increase magnification the FOV increases quite significantly to the point where it will have sometimes greater FOV vs. other scopes with same magnification with higher low mag FOV.
So the moral of the story is: look for the values at the low and high end of magnification and then run them through the reciprocal formula. If the minimum magnification shows that the high magnification is the same as listed by the manufacturer, you "should be" good to go for anything in between with the formula (from either end).
If the calculated FOV at max magnification is different than what the manufacturer states in the propaganda sheet as in the case of the ATACR, use the maximum magnification values for your mid to high magnification calculations and use the minimum magnification values for your low to mid magnification calculations. Or buy a proper riflescope.