I was confused about this, and I couldn't find any information about this matter.
It might not be a big thing, but I wish this could be a good tip for others.
So Mail Armor has DR reduction calculation orders as below
Normal :
BASE 9×1=9 SLASH 9×1.5=14 CRUSH 9×0.5=4.5
Exceptional :
BASE (9+4)×1=13 SLASH (9+4)×1.5=19.5 CRUSH (9+4)×0.5=6.5
Exceptional + Crush Proofed :
BASE (9+4)×1=13 SLASH (9+4)×1.5=19.5 CRUSH (9+4+3)×0.5=8
Consequently, 'Proofing' enchants are always applied at first, so 'crush proofing' your mail armor might not be a good choice.
CRUSH-Proofing on mail armor only increases 1.5 CRUSH-DR, while SLASH-Proofing increases 4.5 SLASH-DR.
If you want to defend against CRUSH damage, you'd rather use other armors, like brigandine or plate.
http://pillarsofeternity.gamepedia.com/Armor