UPDATE since my initial testing –
Rode a 50-mile loop this morning with some off and on hard plus a few steady efforts. Rated Difficult Polar Breakaway Specialist Ride when done.
Viewed activity details, selected View on Strava, three PR segments were listed, selected “View all results”, scrolled the list and starred a variety of segments from less than a mile (sprints) to 20 miles (rolling), returned to Xert. selected Advanced Stats, Refresh and one by one worked through the list evaluating the BTE predictions.
All nine entries look to be reasonable estimates and perhaps attainable considering the goal is an all out BT effort. 
I also went back into Strava and compared the predicted BTE times to the leaderboard for my age group to confirm viable times. Curiously the age group dropdown on the Strava website stops at 55-64 although you can select By Age (65-69) on the phone app. 
In my case, if a BTE segment attempt was successful, I’d be shy of any QOM times shown (forget about KOM 
) but I expect that at my age.
I did notice a slight bug. At one point there were more than nine segments listed (some were dupes), then there were less than nine. But things sorted themselves out by clicking Refresh.