Necro-bump to say I learned something new yesterday:
On Cannondale Hollowgram the arm aligns to one of the dips between the lobes on the spindle splines, both DS and NDS, so both sides are in-phase (the bumps align).
Sram, RF and Easton align the DS arm to a bump, so 22.5 degrees off from Hollowgram. Sram 30mm chainsets have the spindle "permanently" attached to the NDS arm, and although I can't find a photo of the RaceFace or Easton NDS separated from the spindle, RF sells spindles. RF spindles have both sides in-phase, so NDS must also be bump-aligned.
Most Stages power meters are sold as modified left arms, so either you send your left crank arm to Stages and they bond their strain gauge to the inner face and calibrate it, or they sell you a boxfresh arm with the gauge already attached, to match your crankset. For SRAM/FSA/RF/Easton, Stages sells its own carbon arm rather than adding their gauge to an OEM arm: https://stagescycling.com/en_gb/gen-3-stages-power-l-stages-carbon-power-meter-for-30mm-sram-race-face-next-sl-fsa-386evo
The Stages 30mm arm is dip-aligned, like Hollowgram, so the Stages 30mm spindles have the two ends out of phase with each other. https://stagescycling.com/en_gb/stages-30mm-spindles
If you used a Stages spindle with two Hollowgram arms, or two RF/Easton arms, you'd be cantering. If you used Stages or Hollowgram on one side and SRAM/RF/etc on the other side, the arms would be at 180 degrees like normal.