1. a lot of dishes are 22º or 23º offset. Especially when you look at all the DBS (pay TV) dishes.
The commercial offset dishes seem to be in the 22º to 26º range, but that's by no means a comprehensive review.
You should be able to find specs on any particular dish, and go by that.
2. yes, except . . .
The whole reason the dish elevation is off in the first place, is because the LNB arm is bent.
If that is so, using an inclinometer on the dish won't work , either.
Another reason for the elevation being off is due to a home-made LNB mount.
One which places the LNB too high or too low.
And an inclinometer won't help with that, either.
One way to check a dish, if you really all that concerned, is to put it on a plumb pole withOUT a motor mount, and aim it at any one bird.
Check the displayed elevation against the known elevation for that bird.
Then, either mark your mount with the error, log the error, or tweak the LNB arm/mount, as you see fit.
If you really cannot find any FSS birds for your adjustment, you could always take the coward's way out ...
Put on a circular LNB for the DBS (pay TV) service.
And then if you still cannot find one of
those birds...

.