Average Fill Factor is the average of fill factors of a user's contributed solutions. Fill Factor for each solution is the percentage of non-zero marks on a solution's autocorrelation matrix, which can be calculated using the following formula:
Fill Factor = (MARKS * (MARKS - 1) + 1) / (((2 * Game_Rows) - 1) * ((2 * Game_Cols) - 1)) * 100, where MARKS is the number of marks on the original game grid, Game_Rows is the number of rows on the game grid, and Game_Cols is the number of columns on the game grid.