900 is not low for a diesel to idle at, at all, in my opinion.
The new TDI's idle at 900.
If it idles (and runs) properly at 950 i wouldn't change anything.
Most cars idle at 900 or 1,000 rpm except for the newer cars that will idle at 1,500-2,000 for a minute or so on a "cold" start [like parents' subaru].
(Edit:gas cars)
To the best of MY knowledge anyways.