Well...
After discovering how useful is the DES page, my life became much easier !
Just insert a point like the OM/CROSSING ALTITUDE there and watch the magic happens. No more constant position updates or mental math to discover how you're on the profile. The fmc will tell you continuosly what is the required ROD to the point you previously inserted.
Remember that in a clean config, without speedbrakes, the acft's rate of descent is something close to 1100ft/min.... 1400 using the speed shaker !
So, if you see anything more than 1000ft/min there mind that you either will use the speed brake to slow down during descent with that rate, or you can level off, slow down to the flaps 5 speed and descend with the new calculates rate for that config.
I don't know which - 800 do u fly, but in some variants the slats will go to the full extended position with flaps 10, and that, my friend, works like a charm.
By the way, it's very rare, at least where I fly, to see pilots using this helpful tool.
Fly safe and remember, never blind follow the atc vectors, if you're not confortable, say you're unable !