How is total % complete calculated?

I guess that depends on how you interpret it.

In your example, yes, you may have completed 80% of the map/nodes in your hypothetical city, but you will have actually completed no streets - so the 0% of the streets statistic still holds.

This was an idea floated some time ago, so you certainly aren’t alone in your thinking. I agree with the % of the streets completed method of calculating the metric, though.