Thanks @TwoBigWheels. I will try the Slope mode on my next indoor training session.
I did manage to get a BT outdoors towards the end of a 5 hour ride by chasing someone up a hill for 5 minutes. This was good fun, but I feel I probably shouldn’t be trying this hard on an “endurance” ride, but pacing myself more consistently if I want the best overall time.
I’ve read read this thread which seems to be describing the same problem. So I guess my question is more how to get the best out of Xert for multi-day race training?