David,
Background on the eclipse-chasers calculator.
When the calculator was first written by Bill Kramer, there were some major problems with the extinction model being used which I pointed out to Bill. Calculating extinction isn't easy at the best of times because the number of air masses you "look through" is only ever an estimate. Pollution, humidity, aerosols all contribute.
I corresponded a lot with Bill about this and we iteratively tested the calculator against as many of our own real past exposure records to give as good an indication as is possible. We tested the extinction model against results from low altitude eclipses as well as the non-extincted exposure model against our results from high in the sky corona imaging.
The 15 degree extinction exposures correlate exactly ( surprise surprise) to my images from China in 2008 when I was at a sparsely populated non-air polluted site in NW China. The sunset brightness looked a bit like our sunsets here with the sun bright on the horizon not fading away 15 degrees above the horizon as happens in most cities in the east of China.
That doesn't answer your Q about diamond rings.
There is no one correct exposure for diamond rings. For starters, it depends how much photosphere is showing. The true diamond ring is the very last little ray shining through the last valley on the limb. But it's quick and hard to catch.
Diamond rings can be given a very wide variety of exposures - just depend on what you want the thing to show. You can (I'll call it over expose) to get a big spread like the image at the bottom of this page :
http://joe-cali.com/eclipses/PAST/TS...YA_Photo1.html
exposure details are on the photo
There are three images on this page ( about half way down the page )
http://joe-cali.com/eclipses/PAST/TSE2010/joe.html
Ignore the exposure details of the one through cloud showing shadow bands. The extinction by the cloud makes this exposure data irrelevant.
But the first is taken at prime focus of a WO M70 refractor f6.3 1/500s ISO100
The third image below is taken by Bengt Alfredsson. He has deliberately stopped down the lens to f8 to give the diffraction spikes. About 2/3 down the same page are 4 brackets in rapid succession from 1/500s - 1/60s ISO 100 70mm f6.3.
Some people go extreme like this one by Alex Birkener
http://www.astronation.net/index.php...3#!prettyPhoto[Sonnenfinsternis]/16/
I'm pretty sure Blende 14 means f14. 0.3s seems awfully slow for that image even at f14. No time now but one of these days I want to email Alex and check if his settings are correct.
There is no one correct setting so don't stress if you see different recommendations they are based on different but valid outcomes.
Joe