Actually you want to measure the amout of light falling on the leaves, so you would meter off of the plant. Also, here's another thing to keep in mind...you want to make sure the leaves are absolutely clean and free of dust, dust robs light like you wouldn't believe...but no one likes to have dusty leaves on their plants, so it shouldn't be an issue. I go around and wipe off the leaves of my guys in the windows once every three days or so.

Yeah...mad scientist...hee hee hee....I haven't tried light metering off of my plants (unless to take the rare photo) to measure foot candles in forever...I prefer to keep my mad science tendencies for the flasking