I would think that the Air fuel ratio might come in handy, though I have never seen one for a diesel, I am sure that they sell them.
In a diesel I think the ratio of air to fuel is 100:1, whereas in a gasoline (petrol) engine it's 14.7:1. So any changes that are made to
your engine will have to make major swings to get rich or lean in comparison.
As far as the Turbo is concerned, you want to have quite a bit of boost, so don't turn it down. If anything, turn it up. I am not sure what pressure diesel runs most efficiently, but the higher the compression ratio, the higher the efficiency. check what others are running and tune your engine accordingly.
"Diesel engine
In an auto-ignition diesel engine, (no electrical sparking plug--the hot air of compression lights the injected fuel) the CR will customarily exceed 14:1. Ratios over 22:1 are common. The appropriate compression ratio depends on the design of the cylinder head. The figure is usually between 14:1 and 16:1 for direct injection engines and between 18:1 and 20:1 for indirect injection engines."
Check out
this Wikipedia article for more info, hope I helped.