I thought about this question a while back, and I think you can judge this question in an analytic fashion using only a few basic assumptions, but I'm not sure whether or not I posted at the time.
The assumptions are:
- Conserve MP from earring is your only Conserve MP item, and that it is exactly 5% activation chance (and not some fraction of a power of two).
- The chance of getting an n/16 reduction in MP cost is uniform for 1 <= n <= 8 (I believe there are tests somewhere showing this).
- You take damage only in increments that maximize the MP recovered per damage (this is obviously not going to be the case, but as long as you only factor in damage of at least a certain amount, you can modify the formula below slightly to account for the average MP gained for HP lost over a longer term after countering against similar flooring in conserve MP procs by magnetic, or you could leave it biased in favor of ethereal to attempt to account for the unquantifiable situations like Judai's above).
- You are maintaining a more-or-less steady level of current MP over the fight, never falling too low to use a spell, nor sitting on full MP (I think most good tanks will find something to do with their MP when they have it in excess).
Then if R is the amount of MP you recover per tick with your typical support, using the formula 1125*R/4 gives you a value, for which, under the above assumptions, the total damage taken over the course of 30 minutes leads to an MP return from ethereal equal to the average saved in the same period of time by magnetic.
As a basic example, if you typically have 7 MP/tick (job trait + refresh + ballad II + one piece of refresh armor), then Ethereal beats Magnetic taking 1969 or more damage every 30 minutes (again using the above assumptions).
In case you wish to rework the formula, it was reached by taking R * the number of ticks of refresh over 30 minutes (600) * the proc rate on magnetic (1/20) * the average MP conserved in a proc (9/32) * the amount of damage taken per MP returned on ethereal (100/3)