"now remember, your S2 might vary"
The construction on the S2s (and all Nikons) was pretty consistent.
The problem here is that the edges of the S2 finder are inexact. Also, how you use the finder depends on whether or not you use eyeglasses.
I just now compared an S2 finder with an S3 finder, a CV minifinder, an SP finder and a 35mm lens mounted on a Nikon F (100 percent field of view). My judgement has always been that the S2 finder fairly well approximates a 35mm field of view, but that it's easier to do this if you shoot with both eyes open (Leica folks can stop reading here), easy to do with the 1:1 finder. In the test I just conducted, I found that, if I removed my eyeglasses and scrunched up against the S2 finder, its entire field of view is very close to the field shown by the Nikon F, but I still had so kind of roll my eye around to see the whole field. (I was using bookcases). All the finders generally agreed on the framing of 35mm lenses ... the F, as expected, showed a bit more field than the RF framelines.
The S3 finder is considerably larger than the S2 finder. It shows (in 1:1 magnification, mind you) a full field of view approximating 25mm. If I didn't wear eyeglasses, I'd be in heaven. Cosina needs a 0.5:1 mag to show 25mm framelines in its upcoming Bessa R4. The main finder on the SP is actually pretty close in size to the S2 finder. For years, I have guestimated 35mm with the S2. And when I'm doing critical shooting with a 35mm lens on the SP, I'll often keep my eye on the main finder and use its entire field of view instead of switching to the tiny minifinder.
An S3 really helps all around. It shows the 50 and 35mm and 105mm framellines together in 1:1 size. I've shot so many rolls of film at this point that I've got a fairly accurate mental picture of the relationship between these lenses. In fact, I find it very easy to mentally imagine a 28mm frame in the S3, which is comparable to imagining a 35mm frame in the S2.
People who switch between camera systems, and those who use less than lifesize finders, will probably have a harder time. About a year ago I got a CV minifinder for my S2, because my main lenses these days are a 28/50 combo, and the minifinder makes shooting the 28 a breeze.