Assuming you get to do the initial (re) encoding yourself:
According to https://en.wikipedia.org/wiki/DVD-Video#Video_data you can encode the video at max 9.8 Mb/s. FFmpeg has a command line option -target ntsc-dvd
which seems to setup options appropriate for typical DVD style encode, though possibly not "optimal" lossiness just a default DVD style encode. You might be able to tweak it to get better like specifying higher max bitrate or higher average bitrate (the "b:v" and possibly "maxrate" options mentioned http://todayiwantedtoprogram.tumblr.com/post/15142587796/what-does-ffmpegs-target-pal-dvd-actually-do and possibly some of the other suggestions) for instance apple's dvd encoder uses a target bitrate of 8 Mbps. When adjusting maxrate you just have to make sure that max + audio doesn't exceed 10.08 Mbps.
The suggestion for using two pass encoding might be helpful as well, to improve quality. Basically, you're going to want to basically use the "highest quality level" that ends up still fitting on the disk you're writing to (that still satisfies the DVD spec, of course) 4.7 GB.
Apparently there is no minimum bitrate really so that's OK.