In photography there is thing called 18% gray, which as far as I have understand is a color on a "gray card", which reflects 18% of the incoming light and when telling the camera to expose to it will somehow calibrate the exposure for you.
However let's say I have a plane with a material in Blender with e.g. a diffuse shader reflecting 18% of incoming light. And let's say I use standard view transform since filmic apparently does this automatically. When setting the exposure what value should the gray have to be properly exposed?