Thanks to Bob (rmbuilder) and Jerry M, I now know the 10.84 cc's number is the NHRA Stock Eliminator spec for the '69 Z28 302 piston dome volume.
In the meantime, I performed a true cc test on my '69 302 block/piston since my engine is currently apart. To make the readings as accurate as possible, I applied pieces of scotch tape to locations on the piston skirt to keep the piston "square" in the bore. I also wrapped a few layers of 3/4" masking tape around the empty ring lands to make the piston a fairly tight fit in the bore so it would not inadvertently slip down the bore during the cc'ing process. Since my burette holds only 100 cc's, I then positioned the piston "down the hole" 0.400 inches (confirmed with a depth mic), rather than the more typical 0.500 inches just to be sure I wouldn't run my burette dry. The dome height of this piston is just under 0.3 inches, so 0.400 inches down would work OK. This was followed by an application of wheel bearing grease around the edge of the piston/bore interface to seal it off so no water would leak past. All excess grease was then wiped clean. Using a magnetic base dial indicator, I set it on the cleaned deck surface and zero'd it, then re-positioned the magnetic base to put the indicator's probe over the bore. The block's deck was then leveled in both directions by putting wooden shims under various wheels of the engine stand and slightly rotating the block. Once all this was done, I filled the burette with water and then took three separate readings, removing all the water and thoroughly blow drying the piston dome/bore with compressed air between each reading and re-filling the burette to the 100 cc mark for the next test. The burette's exit is position very closely (an 1/8 inch or so) above the deck to keep the water exiting in a stream and not in droplets that would cause ripples in the water's surface. As the water leaves the burette and the level in the bore slowly rises to meet the indicator's probe, the instant it makes contact, the burette valve is closed. With practice, this point can be anticipated by watching the reflection of the probe getting closer to the rising water surface. Very accurate volumes can be achieved. This method was taught to me by Tim Connolly, Chrysler Engineering's one time "Guru" of cylinder head flow. The other method is to use a flat (how flat is it really?) plastic or glass plate sealed to the deck with a film of (unknown thickness?!?) of grease with a small hole through the plate to let the water stream through. With the Connolly probe method, when the water touches the probe, there is a major and obvious disturbance in the surface reflection. The recorded three cc readings for my tests were 71.65, 71.65 and 71.70 cc's, for an average of 71.667 cc's. For a theoretical true flat top piston having no dome, no valve notches at the same 0.400 inches "down the hole", and with my actual bore of 4.002", the volume of the space above the piston calculates to exactly 82.453 cc's. The difference (82.453 minus 71.667) equals the volume of just the dome: 10.786 cc's in my case. This figure makes sense as it comes in slightly below the max of 10.84 cc's allowed by NHRA for this engine.
Hope this helps others to give this a try on their head's chambers some day. A 100 cc burette (non-ISO lab quality) isn't all that expensive.
