By Pandemonium.Isiolia 2009-08-27 10:55:43
Offhand, there are only set tiers, so to speak, for Bio spells, Elemental enfeebles, and certain Enhancing spells. Bio information is on the wiki, and is dependent on Dark skill. Elemental enfeebles are based on INT, and again that information is on the wiki.
Individual equations for relevant Enhancing spells are also given there, and some are conveniently calculated with this: http://nullvector.com/enhancing.php
Otherwise it's not as simple as all that. It's sort of like asking what the tiers are for ACC and STR or something. You can overkill on ACC, but it'll completely depend on the mob being fought, and so on.
Other than that, offensive magic would fall into two categories - things that just land (Silence, Sleep, etc), and things that have a degree of potency to consider, such as Slow or nukes.
Tests done with nukes suggest that the amount of MAcc gained from a point of INT changes depending on the comparative totals. From -10 dINT to 10 dINT, it's a 1:1 ratio. After that, I think it was tested up to 30 dINT and found that it was a 2:1 ratio (2 INT to 1 MAcc).
For nukes, having a negative dINT negatively affects the damage equation. In general one might assume that being in the negative stat-wise is not where you want to be.
While it has not, to my knowledge, been tested in the same way for MND based spells or songs, IMO, it's fair to assume that they function in the same or similar fashion.
For a given spell, you want to have more of the relevant stat than the mob does. Based on the MAcc testing, going to 10 more than the mob has is probably going to eke the most effectiveness out of your gear for spells that you're simply trying to stick.
Past that, it'd come down to gear availability. Can you beat a 2:1 ratio for stats to skill or MAcc? If so, use that. If not, use the skill or MAcc. It's not so much that you don't need anything more, but it may affect what's more effective for you to add.
For things like nukes or Slow series, you want to continue adding things to boost the potency of the spell if you can. So in part, you really just want to test on a given target as to how much accuracy you can sacrifice and still maintain the land rate you need.
While you do see numbers like the 320 (or 330) skill, 120INT benchmark for BLM, this isn't so much a "tier" but a presumed "good enough" range. It's 120 INT because (at least at the time) the highest known mob INT was 110. If you can do better, by all means, do so.