The guide is the ball. Place it where it is when the play becmes dead by rule -- the runner is down or goes out of bounds.
The second part is harder and purely judgment. If a guy breaks free then he might reset the point of forward progress. I wouldn't want to rule a play dead when the player clearly could have broken free and turned the play into positive yardage or even a score. That's the plus side of the risk that comes with breaking free and establishing a new spot of forward progress. The downside is the risk of losing more yardagge as the spot of forward progress is lost. The judgment comes in whether he really broke the tackle or not. Every play is different and there's no piece of guidance to point to and say "this is how it's called."