Dean,

but how do I know that that
particular EGT is the highest reading. i.e. is it taken off an
individual exhaust outlet or further down the exhaust (i.e.
common/averaged reading).


You don't. But again: You are not looking for the highest value in EGT.
What you are looking for is the first cylinder to reach peak,
regardless of the value it is at. The Deakin columns make all this
clear (after a while...).
The only way to make sure is to install a multi-probe EGT instrument.
Actually, with your kind of engine, I would recommend having one.

But, as long as you don't have one, using the one probe you have will
put you in the ballpark.

--
Thomas Borchert (EDDH)