i am fully aware that multiple truss rod questions have been asked recently but the question i could not find searching through thread across the forum. here is my question:

is it true that i can check my truss rod my freting the first and last frets and the gap at the 8 fret should be about the size of a business card and a piece of paper stacked on top of each other?? This is what i have heard but do not know if this are true.

thanks for any help.
i have a guitar set up book and thats what it says. but you'll know just by looking if your truss rod is out when i got a new guitar a while ago i noticed straight away even though it was a tiny amount