Interesting thread this one. I just read in our chief pilots monthly about this and we are told that our CDA rate is 69 - 74 %. But then I always thought the minimum descentrate in the TMA is 1000fpm and a snappy decent helps ATC out.
I think you 'll see our average going up quickly after this info!
And anyway, flying a smooth and stable CDA is much more satisfactory then those stepdescents.