It has been a while since I had that combo but I vaguely remember I didn't have to change the setting Vs the stock screen. Or it didn't really make much difference.
If you measure off a wall or other uniform area you can test the settings and see what they do. You can verify Vs a digital camera or the stock screen then.
I also think it should be 0-1. In the original manual the Ec-N and Ec-R are the only ones shown with the 0-0 setting. The Ec-N is older than the 1V (used with the EOS 3) and the Ec-R is for the 1N RS, an even older camera. The Ec-S is newer than the 1V.
Ec-s is not officially supported by 1V. There is no custom function setting for it. Best you can do is to perform some tests to learn how it affects metering under different scenarios and then apply exposure compensation.
This review reports that the viewfinder gets dimmer by a different number of stops depending on the chosen aperture. That doesn’t make sense to me. I would of thought that a particular texture on the focussing screen would have a constant light transmission penalty at all apertures.
The review does state that the viewfinder is noticeably dimmer. If it was me I would probably start by reducing my iso by 1-2 stops on the 1V. Remember that negative film has a wide latitude on the overexposure side.
