As for the FOV that you set, well what I do is know the True Field Of View (TFOV) of my scope with different eyepieces. So depending on what EP I may be using I set the FOV ring to that size.
I have a number of scopes, and I use them for different purposes. Take my 8" f/4 dobbie. I pretty much use it solely for low magnification rich field viewing. My 30mm 82° EP gives me a TFOV of 3°. So when I'm looking through an app while planning a session and that scope is what I'll be using, I will set the FOV to 3°.
Note also that the TRUE size of nearly ALL DSO's is not what you will see through a scope. The outer edge of most DSO's become too faint to see, and this area may actually form the majority of the DSO. So while an object may be described as being 0.5° in size (like the full Moon), you may only ever get to SEE less than half of that again.
The other "trick" to be aware of is the relationship between quoted size of an object and its quoted magnitude: the quoted magnitude is determined by the entire light emitted by the object over its entire size, and put into a single point source, just like a star. So take the Andromeda Galaxy again. It has a quoted magnitude of 3.4. However it is difficult to see naked eye, and we certainly don't see its entire enormous size. That's because its 3.4 magnitude is spread out over that huge size and it is also not distributed evenly - its core being brighter than the rest of the structure. This is a common problem most people make when looking for objects and can't see them, despite the objects being quoted as being "so bright"...
I did this too and it took me a long time to figure out what was going on
Oh, and don't forget that the FOV ring that you make for yourself can also be used to find out what objects also fall within the same FOV. For instance my 8" f/4 dobbie with that 30mm eyepiece, can fit M8, M20, M21 and a couple of small globular clusters all in the one FOV!
Makes for a busy and spectacular sight!
And of course, different eyepieces in different scopes will all give different True Fields of View. If you don't know ow to work this out, let me know and I'll post the info
Alex.