Below ~8000 resistance, precise is always better and above ~16000 resistance nirnhoned is always superior. Also having a high critical chance favours Nirnhoned while having a high critical modifier favours Precise. Do note that this result is for PvP where the denominator of the Mitigation formula contains 66000. In PvE, this should be replaced with 50000 which means that the threshold when Nirnhoned is always better is lowered. The resistance of some bosses in 2.1.x can be found at [2.1] Project Resistance.
To help with your decision between Nirnhoned and Precise, I have included an additional calculator in my latest spreadsheet. It uses the critical chance, critical modifier, points in spell erosion, focus, and monster resistance to determine which is preferred.
sorcerer