I"ve been on the -400 for over 4 years. I had the worst ground school in the history of aviation.
For me, I trust the FMS decent calculations when i am confident that we are flying whats programmed in the box. When I am off the programmed decent/arrival, I revert to FLCH and ALWAYS keep the 3:1 calculation going in my head. There are times i do not know what the VDI is looking at, so; back to the 3:1.
All that fancy stuff is a tool, not a scientific given. Use it as much as you can, but trust it as much as you would a cop. Blame no one or no thing except yourself, if you screw up the decent. The 3:1 always works.
Merry Christmas, Enjoy the Egg Nogg!