Constraints Manager DIFF pair length match between p/n

 Dear all,

I am using Allegro 16.5.I need to match the etch length between P & N of diff pair in the pcb editor.

Regarding this i want to set the value in constrsint manager.The match tolerance/difference is 1 mil.

Example: diffpair_1 is running  around 1000 mils.Here i want to match the etch length of P (+) & N (-).

Which one is travel very long either P or N ,based on that i want to match the smaller one with the 1 mill tolerance value.

If the etch length ,P = 1000 mils means , N = 999 to 1001 mils

Regarding this 1 mil tolerance value ,how can i set in the constraint manager for all diff pairs by using formula or any other method.

Thanks in advance....!!!!