Anyway, I updated my post to convey that. Just to reiterate, Thunder 6 in this OA set produces 1000 TP which enabled a Myrkr use, and that returns ~800-1000MP. So, yes, the next effect is free Thunder 6 (My Myrkr at 1000TP is ~850MP, so almost double MP returned from this combination).
I actually have this "OA Thunder 6 + Myrkr" combination saved as a toggle that I call "Free MP Button"
Playing around with this today, with no buffs/food that would affect the numbers, my results are pretty much in line with Felgarr's. I am getting a 1137 TP return from a Thunder VI, which gets me a Myrkr for 906 MP in my current WS set (which is pretty good, but does have some room for improvement). Thunder VI cost is 437MP, for a net gain of 469 MP if I'm actually spending the MP.
Now, some of the real utility here is that you could cast your OA spells totally for free with Manawell or Manafont and then it's just pure MP refill with whatever you get back from Myrkr. If I'm using Manafont for any reason, I may start tossing an Occult Acumen powered Thunder VI in at some point during the Manafont window as a matter of course (unless I have zero MP management concerns) to get a nice 900+ MP back for what I see as a relatively minor sacrifice of using a single spell cast for an underwhelming Thunder VI.
I tend to have a habit of hanging onto Manawell for Impact if I don't have another specific purpose for it. But often I'm in situations where Impact isn't that, well, impactful. In those cases, I often just keep Manawell as an oh ***button for one big nuke for free, but TBH if I'm having concerns about MP it's probably better to make that "free" nuke be an OA Thunder VI and have that one Manawell spell give me back 900mp.
Also interesting - I managed to make some small improvements to my Occult set so that it will will also return 1006 TP on a single
Blizzard VI (386 MP cost), which I can turn into 857 MP with a Myrkr. For me that required all of the OA gear, including that I was coming up just slightly short on TP (996 TP or something like that) so I went back to Oseem and upgraded my two OA+10 Merlinic pieces to get them to the max of OA+11. Fern stones are needed for an OA+11 roll, I managed to get all three pieces with 11 in something like 200-250 fern stones, not too bad. Squeaking out some additional STP from slots like JSE cape (or maybe you have Lehko's ring) would also be possible options.
If really low on MP, a single Blizzard also being able to generate enough TP to WS even opens up the possibility of doing two rapid-fire Myrkrs back to back under Manafont: Thunder VI -> Myrkr -> Blizz VI -> Myrkr = 1763 MP total return for my sets, at 0 MP cost (just the opportunity cost of doing spell+WS twice). If you do both nukes (resulting in 2142 TP for me) and
then Myrkr, return is "only" 1397MP, so you do get some more bang for your buck doing two separate WS instead. Up to you whether spending the time doing another cast and WS is worthwhile, or maybe just the spell x2 and a single Myrkr is enough to get you close enough to capped MP that you're happy with it (especially if Ballads or something help make up any small gap). I view this as highly situational and probably exceptionally rare that I'd need it, but cool trick to have in my back pocket and I'm glad I have my sets ready for it.
None of this is like, a foundational part of any BLM strategy I'm using. But it's a nice little extra way to squeeze out some extra MP if needed.
Sets I am currently using:
ItemSet 402041
* Merlinic Body/Hands/Feet have Occult Acumen +11 (which I did need in order to get Blizz6 to a 1000+ TP return, but Thunder VI has considerably more flexibility)
ItemSet 402042
* Amalric+1 all have MP+80 augment. 2451 MP total.
I am aware my Myrkr set has some fairly small improvements available (see some at
https://www.bg-wiki.com/ffxi/Community_Black_Mage_Guide#Melee_&_Weaponskill_Sets). I don't particularly care to deal with inventory-1 for stuff like carrying an additional "BiS" item only to get an MP+10 gain over the Ghastly Tathlum or Etiolation Earring I'm carrying anyway. Similarly, I don't want to pay 4.5m+ to upgrade to AF+3 hands for MP+10 (or grind for AF+4 for 10 more).