It is hard to say what may actually be causing the problem as seems like it could be a defective DVD+RW, issue with DVD shrink, DVD-ROM compatibility issue or a dirty disc suface.
For a start, have a close look at the surface of the DVD+RW to make sure there is no scratches, fingerprints, etc. near the outer edge of the disc (unless this 80% issue occurs with several different DVD+RW's). From my experience, DVD+RW discs are very sensitive to scratches.
To determine if DVD-Shrink is at fault, try copying the VIDEO_TS folder on the DVD+RW to your PC. By default, DVD Shrink authenticates and decrypts the disc and re-encodes the video such that it can fit on to a single layer DVD, which means that if you simply use DVD Shrink to copy the full disc to your PC, it is effectively the same as simply copying the files from the VIDEO_TS folder to your PC since the disc size is already single layer and has no encryption. If this folder copies over successfully, then the issue is with DVD Shrink.
While most recent DVD writers and DVD-ROM drives read all common formats, I have experienced issues reading DVD+RW discs in early DVD-ROM drives, particularly with older manufacturered DVD+RWs. Also, as most Lite-On DVD recorders tend to write poor quality DVD+RW recordings, it would be worth giving a DVD-RW a try to see if this helps.