A method includes i) identifying a period of operation corresponding to a fuel supply requirement; ii) determining at least one ambient air condition in which the machine will operate during the period; iii) determining a duration of time in which, whilst in the ambient air condition, it is required to achieve a predetermined vapor trail characteristic; iv) determining a resultant fuel composition for use by the machine in the ambient air condition to achieve the characteristic, where the resultant fuel composition includes at least one of the first and second fuel compositions; v) determining the ratio of at least the first and second fuel compositions required for sufficient resultant fuel composition for the duration of time determined in step iii); and vi) producing a first signal indicative of the ratio of at least the first and second fuel compositions required for the duration of time determined in step iii).