You may perhaps read Digital Foundry's article again.
Digital Foundry and Gamasutra do pretty good technical articles. Both of their
articles about responsiveness in video games are the best available to the
public. Digital Foundry has even put thoughts into eliminating the so-called
"zero frame" to get more accurate results. Anyway. From their article you can
read that the input lag of the TV in question (a DELL) is 50ms. And if you
read a lil more closely, you will see that they have wrote a similar article
before (referenced in the very same article); Console Gaming: The Lag Factor,
09/2009. If you read through it, you will know how they've treated the TV's
input lag with respect to the results given, i.e. they subtracted the TV's input
lag from the result of each measurement. Trivial.