I suspect that you are not applying the correction for compressibility.
JSA +8 @ FL390 is -55ºC, as you say. That gives 482 knots before correction for compressibility (with a CRP-5 - you may find that an ARC2 gives a slightly different answer).
With compressibility corrected,the answer comes to about 463 or 464.
If your FTO has not shown you how to correct for compressibility, read the little booklet that comes with the flight computer.