Jason,
The limiting factor in how long you can run oil in light duty diesels is rarely the TBN, but rather how much soot you are willing to tolerate. The more soot, the more Fe and Cr wear you are going to see.
I'd keep things simple and use a 1.0% soot concentration as my condemnation limit on an automotive diesel, or light duty diesel pickup (the same discussion applies to those engines). You'll see some minor thickening of D1 @ 1.0% soot, but it will still be in grade.
I've said this before, but I think that Delvac 1 is far and away the best synthetic oil that Mobil makes for the price. Their new MC oils are also very good, but they run $8.00-$9.00 per quart.