I am somewhat new to this board also but here is what I have learned in the last few months. There should be zero PO Failures. Even 1 PO Failure is bad. The maximum PI errors should be below 280. The maximum PI failures should be below 32 (although a lot of guys say it should be below 16).
These are just guidelines. Please keep in mind that the disk quality scans are just one tool to use; the quality scans do not tell the whole story. It is even more important that all of the files that are on the disk can be read correctly.
Although it is unusual, I have had very pretty looking quality scans, however the disks would not play in my DVD. On further checking (using the Scandisk function), I found that some of the files had errors and could not be read correctly (even though I had a great looking scan with a quality score in the upper 90s). Conversely, I have had some very bad looking scans (made with my old DVD writer) that have PI errors in the 1000s, yet the disk plays fine in my DVD player, and all of the files could be read correctly using Scandisk.
So my conclusion is that the quality scans are just one tool that gives you an idea of how well the disk has been burned. However, having a "pretty scan" does not in itself guarantee that the burned disk will play correctly, or that all of the files on the disk can be read without error.