I have to find the exact centre line of the party wall which divides two
semi-detached bungalows. Unfortunately, internal and external changes over
the years mean that simply measuring between the nearest windows on each
property and dividing by two won't necessarily give an accurate result.

Before the two owners get involved with anything more drastic, is it
reasonable to suppose that the external chimney stack sits squarely across
the dividing line? If so, then the centre of the stack marks the centre of
the dividing wall.

Many thanks.