How to find coordinates of the geometric center of a NACA 4-digit airfoil?
I guess you wish to create a halo.

You can use Airfoil Tools' NACA 4 digit generator ( http://airfoiltools.com/airfoil/naca4digit ) to generate your NACA shape. It also features a plotter were you can set chord and halo thickness. 
Positive values for internal halo and negative for external.
Internal halos are interesting to build wing ribbons.

Airfoil Tools generated NACA 6415

NACA 6415 600mm chord with 15mm halo (JPG)



Airfoil Tool's NACA 4 Digit generator