Blog author suggests 36 characters. I have done 50 before but am now tempted to try 36.

Edit 1: I did some quick experimentation with columns that were 36, 40, and 50 wide. There is something magical about 36. With 50 my eye movements were conscious. With 36 it was as if I was able to read the whole column in peripheral vision. Much more immersive. I wonder now about narrower fonts, if they could produce the same effect.