When setting up the Picscan chart it is worth taking an extra few minutes to get things as exact as possible. The chart will become a reference for a number of procedures.
Make sure that the camera is level and not tilted or panned in relation to the chart and that the chart itself is level.
Use a minimum of a (35mm equivalent) 50mm lens and enough light to give you a deep stop with your chosen ISO.
Make sure that your lighting is even across the chart, the colour temperature matches your camera setting and that there is no magenta or green bias.
1. Using Picscan to determine ISO and match meters to camera.
Line the camera up on an evenly lit Picscan chart.
Using a colour temperature meter, make sure that the colour temperature matches the camera setting and there is no green or magenta bias. Try to use tungsten lights for a tungsten chart and HMI lights for a daylight chart. That way any use of CTB/ CTO is restricted to the lighter grades and only minor green / magenta balancing should be needed, if at all.
Mask the chart as shown in the Picscan instructions.
The fist task is to determine where mid grey sits in Raw / log at your chosen ISO. A combination of waveform, false colour and / or zebras should be used to determine this.
Most camera manufacturers list where mid grey should sit when shooting Raw / Log at a cameras native* ISO and this is a good starting point to begin the exposure tests.
Alexa puts mid grey in Log-C at 38%
Sony puts mid grey in Slog2 at 32% and Slog3 at 41%
Red puts mid grey in Redlog at 43%
Canon puts mid grey in C-log at 33%
Panasonic puts mid grey in Vlog at 42%
The most accurate way to expose mid grey is to set the camera zebras to one of the above percentages relative to your camera system and then set your aperture to the point where the zebras appear on the mid grey of the chart. Use a wave form to confirm.
An alternative to zebras is to use false colour. For example, with the Alexa the mid grey false colour (green) sits between 38% and 42%, so exposing just as the green false colour appears on the mid grey of the chart should give you an accurate exposure. Again confirm with the wave form.
Take note of the aperture setting on the lens and set the ISO on your meter to match. This is now your rating for your meter relative to the camera.
Repeat the process in both Tungsten and Daylight. Some camera sensitivities vary under different light sources, so it is worth checking.
* Native ISO usually represents the manufactures compromise between maximum highlight detail and lowest signal to noise ratio in the shadows. Many cinematographers happily stray from this ideal, especially if the need for such large highlight headroom is unnecessary or if the photography calls for minimal noise, such as green-screens.