Your score is computed by dividing the total number of boats you beat in each race plus one by the total number of boats you have sailed against. For instance if you only sailed in three regattas where there were 5 total races and you scored a 2nd of 17, 8th of 17, 3rd of 5, 12th of 13 and 3rd of 13; your score would look like this:
(16 + 10 + 3 + 2 + 11)
------------------------- = 42 / 65 = 0.65
(17 + 17 + 5 + 13 + 13)