By "Dorking Town" do you mean Dorking West? I am confused by your mention of signal RG91 being east of "Dorking Town" (Dorking West?) and then mention of a location a mile after the station (i.e. west) being the next updating point.
Apologies. Yes, by Dorking Town I mean Dorking West; and RG91 signal is a mile or so west (Reading-side) of Dorking West station - too many easts and wests!
As for the first part of your post we'll have to agree to disagree - having been on the receiving end of several "it was on the screens and just vanished" conversations with both passengers and staff I'm really not convinced having it on the screen for several minutes longer would be the best recourse. With the slower running from autumn leaf-fall the "problem" as you perceive it will be worse at this time of year as with a slightly more cautious approach to Deepdene it takes marginally longer from passing Brockham. Through winter, spring and summer the trains should be hitting the sectional running time much more accurately.