Conductor width on a circuit board is not a linear relationship.  If one were to assume a very conservative rule of thumb like 30ma / .001 inch ( in 1oz.per sq. ft. copper ),  the result would provide a very robust design. The temperature rise of such a circuit would always be less than 10 C and there would be sufficient heat capacity to provide heat sinking for most components.  Changing the copper thickness to 2oz. per sq. ft. does not change the current carrying capacity by the same factor.  For 2 oz copper we would use 55ma / .001 inch width for the same overall result.

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