They don't write much about it, in the FCOMs
Does it take into account...
actual airspeed or econ speed, or .78/300/250?
actual wind or fmgs inserted wind?
I think it is quite accurate. It usually coincides with my own calculations within a couple of miles, a few more at the TOD, within one mile below 10,000. If it doesn't it means I haven't accounted for unusually heavy or light or slow or fast or windy.