Ever since 6.04 came out for the 945 LTE I was intrigued by new data fields and metrics. The stamina gauge seemed interesting at first, but I wondered what it would really tell me or if it could help pace a long race.
In short, I think it's kind of useless for Triathlon.
I did a Full Distance race this past weekend and noted after the fact that my Stamina gauge hit 1% with ~30 minutes left of my bike. I then ran well (for me) for about 14 miles before fighting to run and "speed" walk the remainder of the marathon.
In the end my race was a success (for me) and I consider riding for 30 minutes (at the same power I had been for the earlier bike portion) and running for 14 miles to be far greater than 1% stamina.