How far away from the actor do you meter? do you meter his whole face as a whole?
Mental experiment>> put yourself in the position of an actor you've lit - look around and see with your own eyes what light is falling on you - and not just from sources but the total integration of reflections of the whole scene. Now place yourself X distance from this point towards the camera, look around again - is the light the same, or is it perhaps attenuated by the lamp fixtures or set, etc...
Off the top of my head only the sun as a source or for small X can lead to a equivalent lighting (or scenes equating to the same) - this is of course within the constraints of an range/error that you decide, and I guess your question could be interpreted as asking 'what is a good range/error'... Well, thats up to whoever you ask (!) - simple solution as has been suggested, use a spot meter (and try not to think to hard about the angle you hold the card )
...or just place the incident meter right in your talents face. Tape it on