I would imagine that the ATC requirement for no less than 500fpm would also come in to the equation. Something i strive for but not always achieving. As an aside whilst using VNAV the other day it climbed 60ft to enable the speed reduction to 250kts. Try to avoid using it on most sectors, much prefering to use FLCH with speed reduction at approx 2-2500 above required platform. B777 aicraft, how does the bus compare.