This should be editable in descr_campaign_db.xml
Code:
<crusades>
<required_jihad_piety int="4"/>
<max_disband_progress float="20.0"/>
<near_target_no_disband_distance float="4.0"/>
<disband_progress_window float="3"/>
<crusade_called_start_turn float="20"/>
<jihad_called_start_turn float="20"/>
<movement_points_modifier float="2.0"/>
</crusades>
This is at the very bottom of the file(open it in notepad). The line bolded I think seems the most relevant. It seems to suggest that if you're 4 quantities away from the target, perhaps 4 times the units MP, the desertion is nulled. Maybe I'm reading it wrong, don't know, but you could try raising or lowering it and see if the radius for desertion increases/decreases(assuming the line implies there's a radius for desertion).
Cheers,
Augustus