The 12000 divided by capacity is Mark Forsyths's estimate and I agree although only really good packs meet his criteria.
You should get a feel for what is a good IR value although the above figure is a good starting point.
I did a thread on Turnigy nanotechs as I bought two 3S2200 25c packs and they both showed very high IR values which immediately said to me that they could not meet the 25C claim.
Thread at http://www.rcgroups.com/forums/showthread.php?t=1384403
When I tested the packs on a real continuous discharge it became obvious that the packs were only capable of 15 - 18c discharge rates dependant on what temperature rise the user is prepared to risk.
Results at http://www.rcgroups.com/forums/showthread.php?t=1406364
Look at the difference in the IR values for the Loong max, Tipple and older Turnigy packs and you will see that they meet Mark's criteria whilst the Nanotechs are far short. So your suggested calculation is broadly correct, although the 12000 figure is always arguable.