Level 70:1 IS
71:4 IS
72:7 IS
73:10 IS
74:13 IS
75:16 IS
76:19 IS
77:22 IS
78:25 IS
79:28 IS
80
Amp
81:1 Amp, 1 Booster, 1 Seal
82-85:11 Booster (Casting speed of 2), 2 IS (Max)
From then on you max out Amp, followed by Element Composition.
Afterwards you work towards maxing Seal, and since you should skip TS if you're aiming towards 4th job, max out Seal and Partial Resistance.