When stopping is the best move
Leaving at the right moment is worth more than pushing on.
The falling pattern
The score climbs slowly early and drops off a cliff late. The second half of a round is worth much less than the first — unless you built something defensive.
Set a target first
Before starting, set the score you want to reach. When you get there, end it. If the score has dropped twice in a row, it is time to end it.
Note
There is no save button — the score only stays in the current session.