ugh, vessel love. Won't someone please think of the sanitation?
That logic only works if it's a perfect cylinder, which it's unlikely
to be. Middle ground is if it's a perfect cylinder with a curve into
the base, in which case you could measure in a few litres until it's
above the curve and start your ruler from there. If it's got ribs and
dimples, big dents etc like a keg-shaped-vessel or 55gal drum, the
best way to do it is just measure in liquid and mark each level with
an engraver - probably every 5l for a vessel that size. might be best
to do that anyway to save yourself doing calculations in yourhead on
the measurements while you've got wort boiling over, a hop addition to
make and a six pack of the last batch under the belt. obviously sight
glass is best, and mark the outside of the vessel, but inside should
also be okay - pay attention to the engraving when cleaning.
one further thing to consider is whether you're more interested in how
much wort has gone in, how much will come out, or both, ie whether you
start at the bottom, or start at the tap. if the latter, fill until
the levelis above the tap, run it out until it stops flowing and mark
the level as your zero. Two sets of markings are also fine
apologies for any typoes and miserable punctuatuion/capitalisation
above. The PDA isn't designed for this
I live in Parramatta, but I was born, raised, and taught to shout pissed abuse at yellow jerseys from a hill a mite West of here. 
