depends what XDF you're using. if you're using my EEX xdf the bias value is part of the table calculation so it is accounted for in the displayed data. if you change it your timing will have to be rescaled by the same amount. it affects the minimum and maximum spark values, a bias of 20 gives you 60 degrees max and a bias of 0 gives you 80 degrees max. you wont need more than 60 degrees so 20 degrees bias is fine.