I don't have the correct answer, but I do have an educated guess based on what I do for a living as an environmental qualification engineer.
There's this thing called the Arrhenius equation that estimates how the rate of any chemical reaction tends to change in a predictable manner based on differences in temperature. Reaction rate varies with temperature based on a pretty simple exponential relationship -- raise the temperature a bit, reaction rate goes nuts. And vice-versa, cool things down, and the rate slows down. (FWIW, this same general Arrhenius principle can also be applied to some extent to enzymatic activity & denaturing reactions that occur in the mash, oxidation reactions, diacetyl conditioning, etc.)
Fortunate for the laymen among us, myself included, along with the actual Arrhenius equation, in simpler terms, there is also a somewhat well accepted generic Arrhenius rule of thumb which guesstimates that for every 10 degrees C (18 degrees F) temperature rise, the rate of reaction will about double -- and vice-versa, for every 10 C (18 F) temperature reduction, the reaction will happen about half as fast, i.e., take twice as long. The mathematics on this are similar to the nuclear half-life principle (or I'm sure there are dozens of other similar applications in nature).
To answer your question: Since isomerization of hop alpha acids involves chemical reactions at varying temperatures, and since boiling point at atmospheric pressure can be looked up (Google it) based on your elevation above sea level, then for every 18 F reduction in temperature below the standard boil point of 212 F at sea level, I would expect the utilization to be reduced by 50%.
So, for example, at 212 - 18 = 194 F, if 194 F were to be your boiling point, you would apply a factor of 50%.
Since we know you are at 5750 ft elevation, if I Google that quick... your boiling point should be 201 F, which is 7/18 of the way between 194 & 212 F, which on a log scale would equate to a multiplier of approximately 1.25 of the difference in the reaction rate from 194 F to 212 F, which would give you a utilization reduction factor of 50% times 1.25 = ...
So, based on independent estimation, I might believe in the 65% number, not the 83%.
And, since is this all just a very rough estimate... I could still be wrong. However, this was a lot of fun. I love math.