Hi Richard,

You're quite right of course. There is an entire field of study in this issue alone. But I would point out that all of these factors are covered by the caveats,

"... the thinnest width of copper you can remove without leaving any stray bits in the channel to short the two sides together."

and

"... the thinnest trace that you can produce."

Concerning either the smallest isolation, or the thinnest trace, one can produce, a whole host of sins are covered in these clauses, including even issues of registration precision.