N1 as a measure of thrust is more reliable than EPR as the Centaurus incident shows.
The amount of thrust is not directly proportional to % rpm though. I seem to recall that on the CF6-50 (approx 52000lbs thrust) the difference between 100% N1 and 95% N1 was about 10000lbs of thrust, ie. 20% of power. This on an A300.
As all big fan engines work the same way and most of the thrust is developed at the top end of the rpm range, I doubt they had anything more than about 50% of available thrust with an N1 of 81%.
Very lucky on that day!