Just hit 61SCH today and decided to test out Geohelix, as there seems to be very little information about it in the 20 page sticky. In Exp so far, its damage has varied wildly, so these tests were done on Wild Rabbits in West Ronfaure for consistency. All tests were repeated at least 5 times.
First off, base line damage without arts, gear, weather, MAB-neutral support job (/WAR), 50INT and 184 Elemental skill was 69.
First test was to add 4 INT to this setup (Tamas Ring at Lv61). Damage went up to 73. The second test was with Shaman's Cloak, which adds the same 4 INT and, additionally 5 Elemental (and Enfeebling) magic. Damage stayed constant at 73. Conclusions so far: INT affects damage of helix spells, while magic skill does not.
Next test was to see whether Dark Arts offers any bonus to Helix spells (the associated bonus to skill has already been proven irrelevant). No equipment, Dark Arts damage was 69. No increase. The test was repeated with Light Arts to see if there was an associated penalty. Damage was 69. Conclusion here: there are no inherent damage bonuses or penalties associated with Light or Dark Arts. All further tests were with Dark Arts up to reduce MP cost/recast.
Next test was adding 4 VIT to the naked set (Bibiki Shell). Base damage remained 69. This was expected, but necessary for testing a Terra's Staff. Next test was Terra's Staff only, damage rose to 79, a 14.45% increase. Conclusion: 15% bonus from HQ staves applies to helix spells.
Next test - MAB+5 via Moldavite. Damage went up to 72. Conclusion: MAB works on Helix spells, though +4 INT offered higher damage than +5 MAB.
The final test was the effect of Sandstorm. Results here were a bit odd: damage went up to 75 consistently, more specifically weather appears not to be a random proc on the potency of helices, but instead a static bonus. Still need to test real weather at some point.
Throughout the above tests, damage was very consistent; two resists occurred both for half damage, otherwise there was no variation in damage. Tried a few more casts with Ebullience afterwards, one set of casts resulted in 96,51,103,103,103 damage. The 51 makes sense, as a half-resist from 103, but I can't explain the 96.
tl;dr: INT, MAB, weather, and staves help Helix spell damage. Skill and other base stats do not (but might affect resists). Additionally, Ebullience is weird.
Much thanks to Remembrance for spamming Haste and Refresh for half the night.
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