This already exists.
What you are calling "Manhattan distance" is more formally called "Taxicab geometry" and the paradox you are describing is called the "Staircase paradox". It is also sometimes called the "Diagonal paradox"
@jeffcliff Huh, the link I sent wasnt to the coastline paradox (though it does link to that paradox as well)
@jeffcliff That is rather strange, its a well known site and doesnt go down often. It is up for me. Might be a problem on your end.
@jeffcliff very strange indeed. Still loading fine for me...