For the very pedantic, its T -79hrs 58min 30sec.
....was shocked when the T-80 hr time slot produced no change to the seat map. It was only 1.5 minutes after the magic 80 hrs that the seat map opened up. It was quite stressful thinking that row 6 might be my destany for a 90 min flight

Yes indeed - for me, T - 80 never works, T - 79hrs 58 mins invariably does, T - 79 hrs 59 mins invariably doesn't - so you're probably right that T - 79 hrs 58 min 30 sec is the sweet spot. I have now adjusted the clock on my machine to show seconds, so I can put your theory to the test.
Having worked diligently over the weekend to fix seat assignments for Mrs tt7 on 3 flights this week (4D, 23D (767) and 4D), I was somewhat miffed when she called me from the plane last night to say that they had boarded .. but then complained she was in the bulkhead again ....