2.4X isn't supported by your firmware/burner so all you have to choose from is 4X, 6X, and 8X.
There is a firmware update available for your burner which may give you more speeds to choose from...1.03 from here ~ http://www.firmwarehq.com/LG/GH22NP20/files.html
It's not required to verify your burns, but it does serve a purpose.