The value of determination is very low especially for endgame content, the only reason determination had any positive effect for tanks was during low level building particularly before you could access your enmity building skills. The FFXIV Tanking Guide from 1.0 talked about how enmity scaled with Weapon Dmg + STR + DETRMN across the game, low level tanks held greater enmity values when determination stacked and that tank couldn't use skills like halon, or had sword/shield oath or with a combo of low level weapon for the mob vs dps with higher level weapons.

If you are already a PLD or WAR @ level 30 you should be focusing on : Weapon Damage> Vitality > Accuracy > Parry > Skill Speed