Anyway, here's the basic rule: "The standard siglum for Old Persian royal inscriptions is by initial letter of the king’s name, letter for the location, and lowercase letter for the order of its discovery; thus, DNa stands for Darius (I), Naqš-ī Rustam, first inscription." (Political Memory in and after the Persian Empire, vii). While this is good information, is doesn't help much, does it?
But, Livius.org has a nice listing, which is the one I thought I had bookmarked.
Table of contents for copyediting stuff.