Just want to say I had luck with these settings for filling up a 4.7GB/120min DVD-R disc (FujiFilm specifically, although I don't think it matters)....

  Cluster size (Bytes): 2048

  Media name: DVD 4.7GB/120min

  Media Size [Clusters]: 2298368
  Media Size [Megabytes]: 4489.000

  File system Size [Clusters]: 512
  File system Size [Megabytes]: 1.000

  Allowed waste Size [Clusters]: 2297344
  Allowed waste Size [Megabytes]: 4487.000

I use Nero, which said my dvd could hold 4489 megs, so that's how I came up with the media size.  Nero also reports the file system size as being 1 meg, and then with the cluster size at the top, I just went with what a CD is, which is 2048 bytes.  And the waste size, i just subtracted 1 meg from the media size for no particular reason.

I'm not sure if it fills up a DVD to the absolute maximum, but in Nero, it usually showed that it was at the maxium  on the "graph" at the bottom, at least for discs that BTTB said were near 100%.  So it works good enough for me...hopefully it works for everyone else.