Looking for Info / Specs on Obsolete Mopar Piston

It makes sense if I understand it. If the reference surface (the quench flat) is x" down, then you computed the 'perfect' cylinder volume as if the piston was a flat top. Then you measured the actual open volume with the dome in place, and the difference was the dome volume of 19.5 cc.

One more measurement is needed, which can be done in one of 2 ways:
Method A is to measure from the center of the pin to your reference surface.
Method B is to measure the distance of the reference relative to the deck when the piston is at TDC.

Method B is preferred, as it will compensate for the block having been decked, or for any rod length changes made. Method B should be easy if the lower end is still assembled.

With this last piece of info, it will be easy to run an SCR computation for you.