Playing with a Rail Driver and using a tap controller wheel can be touchy. I do wish that DTG would set up the BR110 DB like they have with the BR 204 where the entire throw of the throttle lever is devoted to the speed control. On the BR 110 you only have half of the throw and the back haff is not used for anything, so it is available to be used for better control.
I used to map the up and down keys (next to the four way on the right) for tap changing when I played those sort of locos on TSC Much easier than trying to use the variable % levers
I would have another look at that. I seem to remember that you can on some things, not necessarily the throttle though