IMO idle sets are split into 3 sections:
Standard, things you should be wearing in nearly any situation:
* Regen (how much is up to you)
* Refresh (/WHM)
* Fastcast gear (if you don't use spellcast to 'precast' your fastcast)
PDT, where you are likely to take physical damage
* Darksteel set
* Goliard Trews
* Terra's staff
* Jelly ring
MDT, where you are likely to take magic damage:
* Mermans set
* Mermans earrings/rings
So you should wear Standard + most likely damage to make a idle set.
But unless you are in a low man situation or expect to take heavy damage a full on MDT or PDT set will be overkill. Standard set plus a few items you will be carrying any way (Terra's, Goliard Trews and Umbra) will do most of the time.
DEF, VIT and evasion will all be pointless.
Shadow ring is magic, P.ring is physical therefore they depend on what kind of damage you will be taking.
Umbra > Shadow mantle imo (I like the consistency)
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