Volume at Flame-Out (VFO) is a crappy/hard number to measure for many reasons. The wort volume is unstable at boiling point. It swells and carries on. Also, if you use an immersion chiller, you have displacement problems so VFO is useful is some situations but even then can only be a pre-check to...
Volume of Ambient Wort (VAW). VAW is best calculated 'backwards'. In other words...
Volume into Fermentor (VIF) plus Kettle to Fermentor Loss (KFL) = VAW
You can measure VIF and KFL in several ways. If you use a plastic fermentor, these have graduations on the side (sometimes inaccurate

but usually in the ballpark) and kettle trub can be measured by pouring into a graduated jug (also sometimes inaccurate so double-check them with plain water - 1 litre of water at room temperature should occupy 1 litre of space/volume). The BIABacus 1.3T does have a weight method of determining volumes which is a more accurate method for several reasons but only use it if convenient to do so.
Remember, any brew day is not predictable so measuring ad nauseum is a job for people like myself, Mad_Scientist and others who want to come up with the most accurate 'averages' for everyone else to work from. Commercial breweries can't predict the results from a single batch and have to adjust each batch. Our batches are far more unpredictable than theirs so don't be worried if your numbers vary at the end of the brew day.
Your main job is to be absolutely sure of the weights and volumes you put in to the brew.
Your next job is to take enough measurements so as you really
know if something has gone wrong (usually it is just evaporation rate).
Your last job is to make any necessary/informed corrections before you pitch.
I'm drunk, that's it!
PP
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