I had the same question and went to Phil Bradon at Sekonic and Andy at AbelCine. What I got from them was a procedure that allowed me to test this.
Bottom line seems to be native at 0db is ISO 250. One stop = 6db.
How I arrived at this:
I took an 8x11 gray card and lit it with two hot lights, sufficient to give me good exposure at 1/50s at f8 on my Canon 5D MkII. I'm shooting the Sony at 24fps, 1/48s shutter and 0db. I adjusted the histogram on the Canon by changing the ISO until the middle gray spike was centered (50%) on the histogram. This was at ISO250.
Going back to the Sekonic, I metered the light until it read 24fps at f8 and ISO 250. Perfect. Looking at the screen on the camera, it also was right on. 1/48s, f8, 0db.