unfortunately, the Godox AD600B takes a full 1/225th sec to blast out 90% of its full 1:1 flash output (the t0.1 measure) - that means any shutter speed shorter than this will result in lower flash exposures as they capture less of the full flash duration.
at the 1:2 + 0.7 flash output setting, the t0.1 duration is 1/350th sec
at the 1:2 + 0.3 flash output setting, the t0.1 duration is 1/535th sec
timing of the flash with respect to shutter may impact these measurements, but for these tests, I have used the default timings (the Godox AD600 does allow these to be changed!)
at 1/320th second, one actually appears to get approximately the SAME flash exposures measured on the camera's histogram at either full, 1:2+0.7 or 1:2+0.3, which suggests you may as well just use 1:2 +0.3 at this speed
at 1/400th second, one actually appears to get approximately the SAME flash exposures measured on the camera's histogram at either full or 1:2+0.7, and a slightly less exposure at 1:2 + 0.3, so one could probably use either of the latter settings rather than full output which is being wasted.
as one shortens the shutter speed from 1/250th to 1/320th to 1/400th sec, there is a sequential small reduction in the camera histogram exposure and to gain a constant exposure, aperture needs to change by 1/6th EV for each of the 1/3rd EV changes, but the BIG advantage is that by doing this, the ambient exposure is falling twice as fast, by 1/3rd EV for each increment.