Right, doing that got me the 246 number (which is identical to what TR suggested after the test). However, because that best minute included the power drop in the penultimate step that was only due to my computer freezing (the Garmin recording in parallel showed the penultimate step as completed and at target power) got me 252 at best 1-minute-power x .75.
In the end, though, I am probably overthinking this. I’ll just retest after recovery and take it from there.
EDIT: Just for reference, I posted the Garmin data from GC in this thread: App froze during ramp test - #5 by cyclhist.