Thats the problem we ALL have a number we like to
put them in at... being that I worked in the exhaust
lab I tried to get everyone to put them in at a set
distance... which was 1" so we could talk apples
to apples... all the headers I built I put them in
at 1"... plus it was a easy number to remember

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I agree,
I am trying to look as closely as I can at the temperatures seen in the combustion chamber,so the closer the better.A better way would be to look at the actual chamber temperatures but it is an expensive thing to accomplish.
In theory,as long as you check the plugs and get a reference egt number to go with them,then you have a number to aim for after making any changes.

Keith