If the tutorial calls for inches then change your settings to inches for that light setup otherwise your results will be vastly different than what they get.

What tutorial are you doing?  If the link isn't in the Lighting Bookmarks thread I'd like to add it.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi