When playing from the same save, the rating depends ONLY on the number of days.

SCORE = [ R*(200 - [N*K]) ],
where N is the number of days spent,
R is the rating (its percentage value can be seen on the "Scenario Information" screen in-game),
K is the map size coefficient, equal to 1.4 for Small, 1 for Medium, 0.8 for Large, 0.6 for X-Large.
Once the value of N*K reaches 60, each subsequent day counts as half a day; after
120 - as a quarter day, and upon reaching 180, it stops growing altogether.
If victory is achieved during the computer's turn between days N and N+1,
N+1 will be used in the result.
Rating R depends on starting conditions and map difficulty. For
Easy/Easy it is 50%, increasing map difficulty adds {+20%, +40%, +80%},
and toughening starting conditions adds {+30%, +50%, +70%, +90%}.