Not sure where you got your numbers from but my calculator gives me:

Max. 6500
Avg. 3200
Min. 2400

Mike

A dual layer disk is 8,540,000,000 bytes. If you encode your audio using 192 kbps AC-3, then for 3 hours 45 minutes, and with 1.8 GBytes of existing data already on the disk, you should encode at an average 3,665 kbps (I have a very nice Excel spreadsheet that calculates these things, for any size disk, any audio bitrate, and any amount of existing data on the disk).

Your encode rate of 4,200 kbps is way too high and will not work.

You can encode some things at a lower rate, but if everything is already done at 4,200, you'll have to encode the other items at considerably less than 3,665 in order to make everything fit. Quite frankly, once you start encoding at less than 3,000, the results are going to look pretty bad, even when using 2-pass VBR encoding (which is mandatory at these low encoding rates).

I suppose you could try the trick of creating the DVD, creating files that exceed what can actually fit, and then use DVD Shrink to reduce the size of all the files. However, while this trick works pretty well when you are only a few percent over what will fit, in your case, a lot of shrinking must be done on files that are already going to be marginal. Despite this, you might want to give it a try. DVD Shrink only takes a few hours to run, even in its highest quality settings.