Just for interest sake, I used a PC-based aviation calculator to arrive at the following:
At FL390 with a TAS of 499 one would expect the Mach No. to be 0.87 with an ISA atmosphere, resulting in a TAT of minus 23.7 C. (M 0.84 at FL 390 with ISA would produce a TAS of 482 kts, and TAT minus 25.9 C))
To achieve M 0.84 with the same conditions appears to require ISA + 16 C, thereby giving you a TAT of minus 7.7 C.