jtr
At max. t/o weight, achievable cruise level is FL310 (east) FL320(west). At these levels and at crz power, the fuel flow is in the order of 3,300kgs per hour/ per engine i.e 13,200 kgs/hr-which is where my approx 45mins comes from. It is not beyond the realms of possibility that at M0.86, the g/s with a good wind "up the chuff" could be 600kts-hence the 450nms. But if you prefer to consider the extra 10,000kgs from the HST to be usable at the end of the cruise, say FL400 and M0.845 when the fuel flow would be 8,000kgs/hr, then that 'extra' 10,000kgs gives about 1 hour of fuel depending on headwinds/tailwinds etc. -which is why I used approximately in my earlier posting.
However, the question originally asked why 747-400F did not use a stab tank (whether fitted or not) and I stick to my original guess, that loading 10 tonnes of fuel in the stab. may make trimming the a/c impossible without restricting the load and it's distribution.
By the way, a recent Boeing modified software programme prevents HST fuel from being used until total fuel reaches about 136,000kgs, when an eicas prompt is displayed to switch on the stab pumps.
Hope this clarifies the situation!