Regarding moon phases:
It takes 80 hours for a full moon cycle. For the Artemis' Medal to be better than the Goetia Chain, the moon must be at 25% or more (Matck +4, Macc+8).
The moon phase is below 25% aprox. 19 hours of those 80 hours, or 1/4th of the time. This neck is better than goetia 75% of the time.
That is, if it does give Matck +12. Some reports have said Matck+10, and if that's the case, than the moon phase would have to be higher than 30% (give or take) to beat the Goetia Chain.
Just got my Medal, and so I started looking around SpellCast moon phase rules/variables. Based on the posts I've seen, I think a dynamic set works best. I'm lazy and don't feel like carrying Uggalepih Pendant--I don't often reach the latent MP percentage in Abyssea anyway.
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<var name="NukeNeck">Goetia Chain</var>
<if advanced="%MoonPCT >35">
<action type="Var" when="Precast" cmd="set NukeNeck Artemis' Medal"/>
</if>
<else>
<action type="Var" when="Precast" cmd="set NukeNeck Goetia Chain"/>
</else>
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Now, for a magic attack set, use the NukeNeck variable:
<neck>$NukeNeck</neck>.
@Quantico:
action-Tags are not supported anymore in newer version of SpellCast. Also, in case performance is an issue, you should avoid using the advanced-attribute, and since it's not used for anything other than nuking, it can be done entirely without variables as an adjustment at the end of the file.
Note that moon percentages aren't always the same for waxing/waning situations. This equips the proper gear for every situation. Alternatively you could have a third case for Uggalepih Pendant, just do an <elseif mppaftercastlt="51"> block in between the other two.
Also, note that there's such a thing as too much MAB. INT will give higher benefits at some point. 2INT = 1MAB is not an actual conversion, it's an approximate relation.
I believe Spellcast can, I could be mistaken but I know when I use Fernirs buffs a /p macro that uses "%Moonpct%" comes up and says the moon Phase and the current buffs that phase gives. Perhaps this could be used to achive what your after Dasva?