For LS/WF, magic path fern. If augments continue to be mediocre and you run out of fern, you can try taupe.
I'd really suggest using Pellucid as your 2nd best option to Fern. Yes, AGI is incredibly valuable to the math for Leaden Salute, Wildfire, and Trueflight(if you're a RNG doubler), high MAB will certainly out-perform the AGI difference between taupe and pellucid.
BUT AGI is still an "off path" stat for Herculean Magical, meaning you just won't see it that often, certainly not as much as the "on path" base stats of INT, MND, CHR. So not only are you fighting the random number generator of trying to get at least +11 to a base stat (the only time taupe becomes really relevant) while also fighting the off path luck to actually get AGI.
Whereas with pellucid, you've got your best chance for the highest MAB/macc possible while maintaining a cap of 4% WSD possible. You still have the same chance of
any AGI, just a cap of 5 less. Well, 2-3 MAB will out perform that 5 AGI depending on the rest of your gear.
Stones in general are pretty cheap, not to mention its Dragon Campaign right now meaning while Quetzacoatl gets spammed even more will flood the market (or your personal pools if you're doing it!). On top of that, DM can yield some really great results on Herculean gear...some examples I use in various Magical Ranged Weaponskills:
Herculean Vest- Macc+3, MAB+20, WSD+6
Herculean Trousers- Macc+20, MAB+20, WSD+5 (can be met w/ a perfect fern, but I happened to get this w/ DM)
Herculean Boots- Macc+19, MAB+47
Good luck with the pompous *** Oseem!