I totally agree with Duke, I am willing to bet a monthly salary that even in no-wind conditions a vnav descent from FL390 down to minimums (setting flaps according FCTM while letting VNAV decelerate for the approach) will not function properly and will end in a high energy approach.... not entirely reliable
What I do is that i usually enter a descent speed 10 knots lower than what I am actually planning to fly in order to compute a lower angle descent.