If you want to define the apparent acoustic center as the location in space where sound eminates from, it turns out this location is actually an inch or two IN FRONT of the driver (when mounted in a box). This has been determined an confirmed multiple times via this experiment: the on and off-axis reponses are obtained for the driver, in a horizontal plane intersecting the driver, every few degrees. Sound pressure measurements are performed for various frequencies, down to some very low frequency such as 10 Hz. At very low frequencies the system should be acting like a monopole, for which the sound pressure radiates spherically. This means that the pressure and SPL should be equal at all positions around the "center" of the source. When you make such a measurement at VLF, it turns out this "center" is actually in front of the driver by a couple of inches and not back at the voice coil or whatever. I first hear this from Siegfried Linkwitz back in the early 2000s and at the time it sounded wrong but it is indeed the real-world behavior.

Your own measurement shows some delay of a few usec (51.73usec). This corresponds to a length of about 0.7 inches behind the baffle, so a few inches behind the apparent center. This does not sound correct, unless this includes some crossover components, e.g. a highpass filter.

Anyway to answer your question about a "distance", you just need to calculate how far sound travels during the time interval in question. This is simply:

d = c * t

where d is the distance, c is the speed of sound, and t the time interval. The units must be the same for the numbers you plus in e.g. if you use c= 344 meters / second then d will be in meters and t should be in seconds.