Sorry, cap_56, but you can balance a field.
Either you do it in actuality, by means of construction of the airfield such that ASDA and TODA are, in fact, equal - which obviously you CANNOT do on a day-by-day basis - or you can assume the field to be balanced, by assuming that BFLA=min(ASDA,TODA), which, taken in conjunction with data relating field performance to (balanced) filed length requirements - the BFL you are using - enables a simplified assessment of the takeoff conditions to be undertaken.
In fact, if all that is available for take off performance data are BFL (required) then you have to assume a balanced field available because you have no way to take credit for the excess ASDA or TODA, as you have no ASDR or TODR data.