I find 80% or above with any good encoder, such as DVDFab, CloneDVD or DVD Shrink, seems to produce very good results. Depending on the app, even 70% still can look really good on a normal TV, also depending on what options you choose - like single or double pass, deep analysis, adaptive error compensation, interpolation, etc.
Sometimes you can lower the audio quality (by converting to MPEG or something) and that way have room for a higher level of video quality.
I guess side-by-side comparisons, or one right after the other comparisons on your own television with your own video player may be the best.
I know with my Svideo connection, I can spot things quicker than if I use a regular AV connection.
Anyways, my rule of thumb is about 80% or higher if I can.