I am trying to understand how to get perfect exposure using gray card. With my Sony a6500 using ITU709 gamma, I locked exposure by doing spot metering on the gray card. When importing in final cut pro the waveform monitor show IRE value near 25 for the gray card. Shouldn't it show between 40 and 43 for 18% reflectance mid gray?
18% reflectivity means it should reflect 18% of whatever linear light is actually on it. Gamma should compute 117 at 46%, but your picture shows the card to be in some shadows, so then it can't be 18% of the brightest 255 (expecting gamma to 46%). But the histogram won't be the exact computation, because the camera is busy doing other things too, like your contrast setting, and white balance, and your color profile like Vivid, etc. The purpose of those settings are to shift the histogram. Mild settings in the bright normal light should be near ballpark, but probably not exact as planned. It is not an adequate calibration solution.