Two things:

1) I think the "misconduct issue" is out the window. The DA has specifically said he LIED about the supposed misconduct in his interview in the movie. From the latest from that DA, there was no misconduct.

2) The max sentence in 1978 for what he pleaded to was 20 years. So, even if they followed guidelines then, he could effectively get a life sentence. A quick internet search seems to show that the max penalty NOW for "illegal sex with a minor" is 4 years.