ineligible downfield
 

nfhs enforcement dicates basic spot is previous spot, correct?

yes

For what kind of play? For running plays the basic spot is the end of the run.

how could we have a running play with that type of penalty - we would have a legal forward pass that crosses the line of scrimmage and that action, along the action that precedes it is considered a loose ball play - correct?

Whoops. I read only the content of your post, not the title. Yes, ineligible downfield occurs during a loose ball play, and that explains why the basic spot is the previous spot for that foul.

It's worth noting that the type of play, not the foul, generally dictates the basic spot. That makes remembering the basic spot a lot easier.