It is better to use the field stop size. The formula you are using is a reasonable approximation, but calculating the field using the field stop size will give you the exact value. The values below have been slightly rounded.
The field stop of the 30mm is 43mm, and this will give a true field of 4.48 degrees.
The field stop of the 24mm is 33.5mm and this will give a 3.49 degree true field
The field stop of the 18mm is 25.3mm. The true field will be 2.636 degrees
As I said, your numbers are reasonably close, but modern ultra-wide eyepieces usually have some geometric distortion and this will throw the simple calculation you are using off a bit, but as you can see, not much.
(Simple calculator formula is field stop size divided by scope focal length, with the quotient multiplied by 57.3)
Hope this helps, but your numbers were good enough to make the decision.