As you use Filmic for your colormanagment,you want to have the midtones calibrated well.
Its done very easy.First make a new material with a diffuse shader with a value of 0.18 .use this material to your main object.then switch from Filmic to falsecolor and adjust your light strength,untill the keylight that hits the main object is grey.then switch back to Filmic and switch your material back to your PBR or what ever you use. (side node.i dont know how often i have posted this mehod,it should be pinned)
now your have calcibrated your scene like a Photographer with a grey card.
the only reason to capturing diffuse values for your self is if these materials doesnt exists as PBR materials,and you want that “scanned”.
maybe you can give as more information about that.you can find allmost every material in the net for substance painter ect.
these links maybe are interesting for you