It depends on each induvidual engine really. Most diesels cant really go below about 16.8 - 17:1 without producing smoke, so aim for figures around this if you dont mind some smoke.
If you want a perfectly clean tune, go a bit higher, about 17.5 - 18:1.
See the attached Excel sheet. You put in your values in the yellow section (mm3), it converts it to mg in the next table and the third table shows your AFR.
You'll need to make sure the axis (Airflow and RPM) are the same as your smoke map.
The values in this file are the ones I'musing in my tune, making just over 135bhp from a normal 2.0 HDi 110.
AFR calculator for HDi mm3.zip
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