Starting the timer is clear, but in order to have a better understanding on when to stop the timer, I think it would be good to have the rules revolve around the last action done by a player. I have the following propositions:

• Chapter I - activate the dynamite switch
• Chapter II - kill the last enemy in the docks - confirmed by Ray's voiceline "We did it!"
• Chapter III - up for discussion, but I think the start of the cutscene is a good point to stop
• Chapter IV - camera lockout
• Chapter V - defeat Devlin's bodyguard in a duel
• Chapter VI - activate "Head to the meeting with Mendoza"
• Chapter VII - cutscene of the brothers and Barnsby riding horses begins playing
• Chapter VIII - reach the hideout
• Chapter XI - screen fades to black
• Chapter X - screen fades to black
• Chapter XI - cutscene of going down the waterfall starts playing
• Chapter XII - dynamite detonates
• Chapter XIII - bell falls
• Chapter XIV Thomas - defeat the bandit in a duel Ray - reach the objective at end of the tunnel
• Chapter XV - defeat Barnsby in a duel