I know this is a rather subjective question, but as a rule of thumb, what is the lowest quality percentage for a DVD movie you can accept before actually noticing a differencce? Will, for example 85% compression for typical movie be perceivable?

This would be the cut-off point you would use to split the DVD-9 onto 2 DVD-5's instead of compressing any more.

I don't think that you'll see any difference between 100% and 80%. There are probably some differneces when you use a high quality TV but not on a standard one.

Personally I use a LOEWE 100Hz TV and I don't see any differences at 80%.

It also depends on the movie content ie high action/complex scenes. I did a 3 hr. movie (Snow Queen) IIRC app55% compression with excellent results

If I am using CCE 3-pass to compress I find I can take it right down to around 40% and it still be very good quality. The data rate then is normally around 2500kbps, but that is higher than most SVCD's, and SVCD's still look pretty good if encoded properly. I find however with a 1-click ( CloneDVD, dvdShrink) that the quality is poor under about 70%.

Ben