I'm working with the best setting of the x264 files at the same time, but I can only have 1 answer: try it by yourself. Different person have different expectation for the quality of the video. It is hard to say the exact setting. But Fairuse wizard does handle most of the settings for you. What you should do for it is to look at the target file size and click 2-pass encoding, then you should get the best result with the files size you want.

You can simply just calculate how many movie you want to store in 1 single DVD and you will know the file size for each movie.