The test was done with the proper grade reducer at each temp range.
You are right longer flash is better as most tech sheets are written for production type work.
Also on the math, it would be done this way, not clumped.
70 deg = 5 minute flash time.
60 deg = 5 divided by .85 =5.88 flash.
50 deg = 5.88 divided by .85 = 6.92