[Varyon]

It fails with attacks that barely penetrate extremely high DR. As such attacks would still cause pretty severe wounds using difference of squares, and as such errors have a fairly small (relative to the full damage) range where they occur, I'm alright with overestimating them if it means only having to do simple arithmetic at the table.

As for the error on the far end, you are correct that I'm fine with it largely because, at that point, the character is almost certainly dead anyway.