jp80874: Anti-newton ring glass is a whole other can of worms, although it does appear the stuff from focal point is better than some of the other anti-newton ring glasses I've experimented with. But the main reason I didn't mention it is because I'm talking about the glass below the negative, which would not be anti-newton ring glass.
In the end though, since I had mostly been unhappy with anti-newton ring glass (above the negative), I came up with my own solution for newton rings. I use the same glass above and below the negative. To avoid newton rings, I use a piece of Tri-X 320 (unexposed, undeveloped, fixed) between the top glass and the negative. No more newton rings.
artonpaper: For 35mm I still use my home made carriers with the filter glass (I had also added registration for masking). The glass that originally came with the carrier had some blemishes (even though it was new) so I threw it away at the time. Means I can't do a comparison. At the time I had also read the use of coated glass above the negative could potentially reduce the likelyhood of getting newton rings so that was another incentive for me to try filter glass. Since newton rings are an intermittent problem, I have no idea whether the coated glass is actually helping or not in the respect.