-found this pprune link......
http://www.pprune.org/tech-log/34016...le-factor.html
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So, . . . how about half your groundspeed likein conventional flying and work it out yourself - that way you can keep abreast of the descent profile . . .
what about the semi circle (on the ND) that - if you adjust the RoD the semi circle moves forward or back corresponding to your selected V/S.