Just my 2 cents...
You did not quite understand the difference between those two posts saying about starting with the 40mm focal length (that regards your zoom lens) and employing the 50mm prime lens (which is a different lens).
The lens is certainly of utmost importance and I agree with everything that neuroanatomist has said. Suggestion that you use a prime lens instead of your kit zoom lens is a valid one, but not critical - if you use neuroanatomist