For a SRAM Red I get 26.7mm from 1 to 10, so that's 26.7 / 9 = 2.97 mm/shift

Measured by placing a clamp on the cable below chain stay and measuring the distance between the clamp and cable stop with calipers. The measured positions were reproducible to 0.1mm. The derailer stops were loosened to prevent them from influencing the measurement.

Since the cable is more taught in the big cog than in the small cog, the cable travel at the shifter is probably a bit more than measured at the chain stay due to elasticity in the cable and housings.

Measuring the face to face distance between cogs with the caliper I get 20.2 mm for 6 or 20.2/5 = 4.04 mm/shift so that gives an actuation ratio of 1.36:1

Kind of splitting hairs here.