Did test with determination as well this last weekend.
Not large enough to be exact figures, but in the test with drastic changes in determination we expected to see some change... it was literally an average dmg change of 0.14% per nuke average from +0 determination to +36 which is easily in the margin of error. (a few points of dmg more or less)
Used gear without int and to test DET increases to gage if there was a dmg increase or not. (did a sample of 30 nukes per each setup... yea not large enough yet but just started towards the end of last beta)
+0 +5 +10 +15 +22 +36
For each of these we shown almost no correlation to dmg output from thm using only blizzard 3 on level 31 mobs. With int, we saw drastic increase per added stat per test.
So I am not sure if determination effects dmg in another way but for now it seems determination has no impact on dmg output for mage nukes. Was going to test and see if it effects DoTs or maby some other factor in the dmg output. But int is vastly superior to det based on initial testing.
This will be important to know when materia comes into play with the low stat caps for materia gear. Most mage gear can only hold 1 int materia. So need to see if mp, spell speed , det, or Crit will be better.