It can be accurate , but it is better for checking valve clearnce.
Just CC your heads with water by covering the chamber with a stiff piece of plexiglass that has a fill hole drilled in it. I used an old CD ,it was a clear one that was used as a spacer or in a pack of new CD-rw that I bought.
I bought a Beret from Online science mall on e bay for like 25 bucks.
It was easy to check, and accurate once I had the right stuff.
Some guys mix something with the water to help break surface tension, so it gives an absolutley perfect reading.I didnt use anything but water. I just needed to know I was real close.Got both heads shaved and ended up at 82 CC s on each one.I checked all 8 to be sure they were all even before i had them milled, just to keep the record straight.
Fact is stranger than Fiction