I just purchased the MyDVD 6.1 DVD authoring software and I found the function "shrink-to-fit" very useful.
As you add movie files to your project you will see a dial moving from High Quality and down as the program figures out the best framerate in order to fit the movie files on the disk.
I have not experimented too much but I put three 30 minutes MPEG files from my TiVO on one disk and the quality was good.
I thought I should post this as there are many threads on this forum about how to fit maximum data on the disk. This program makes it very convenient.