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.
If I assume a bull to be a cow then I could milk him.
Twisted reasoning to prove a point that has no relevance.
I realise I am involved with some Brits that can not accept a simple truth.
I just hope that the chap that raised the question does not get completely confused by your assumptions.
Cheers and good luck with your B-Levels