There's one thing that you can do for yourself as a calculator. An EP will have as a spec its Apparent Field Of View (AFOV), and depending on the scope you are using a certain magnification. With this youcan work out its True Field Of View (TFOV), which is what you want. The equation is simple:
AFOV ÷ Magnification = TFOV.
As an example, let's take an EP of 30mm with an AFOV of 68°, and a scope of 1000mm focal length (aperture has nothing to do with FOV, only itsfocal length).
First, the magnification this EP/Scope combo gives is 33.33X. Then, the TFOV = 68° ÷ 33.33 = 2.04°
This is roughly four times the size of the full Moon.
Now, same scope, but a 5mm 82° EP:
Magnification = 200X. TFOV = 82° ÷ 200 = 0.41°
I occasionally use a calculator of sorts with Sky Safari. It has a function where you can input the size of the TFOV an EP will give, and the app provides a fine circle of the size of the TFOV. But I only use it as a planning tool, not in helping me decide what EP to use. This is because wirh different objects there are many factors that affect which EP is most efficient for it, seeing, transparency, magnitude and its size (some objects may be described as "bright", but if its size is large, what you see through the eyepiece will be dim), and also here un the field, aperture (how much extension of the DSO you can actually see).
FOV calculators can be useful, but not a panacea for in the field eyepiece selection. Try and error and experience will be your best eyepiece selector when push comes to shove
Alex.