Waldos or Lovecraft?

The History of 420

As you know, the group of friends nicknamed the Waldos have been accredited to coining the term "420,"  but where did they get that time?  I don't believe it is just that it took them exactly 20 minutes to leave school and meet up.  Why not 4:15 or 4:30pm?  To me, 4:20pm seems too specific of a time for a group of high school boys. My suspicion tends to conclude that they read "In the Walls of Eryx" in school (see the hyperlink below) and thought it was funny to reference the time for themselves.   

"In the Walls of Eryx" by H. P. Lovecraft with Kenneth Sterling (hplovecraft.com)

My next question then leads to why 4:20pm for Lovecraft?  This is a longer conversation best not to write over email.  I will say it all ends with Horus, the Egyptian God, where Hahsish is first discovered and recorded in the 12th century... And my parents thought majoring in Medieval Studies was a waste of money. :)