Is there a minimum thickness for the deck itself to measure with sonic tester? going to 9.75 is only .025 off when you list stock can be .035 making it not even zero deck. 
Seth

The nominal deck height is 9.775. Ted Eaton routinely surfaces the block decks for a final deck height of 9.75. If using composite head gaskets, like Best, you should have .030-.050 to work with between the block & heads before intake manifold alignment becomes a problem. If you have non-posted ECZ-G heads, Ford recommended limiting surfacing to .010; others have said that they're safe up to .025. My non-posted ECZ-G heads have been surfaced .013. For each .006, the chamber size is reduced 1cc. If you don't have non-posted ECZ-G heads, you can safely go more than .025. Keep in mind that the original cast pistons were about .020 below deck height; combined with the original shim gaskets (.025), the quench was .045. If you are using aftermarket cast pistons, they could be as much as .035 below deck; combine that with the compressed thickness of Best gaskets (.046), your quench would be .081, which is excessive. Zero-decking the block would provide quench of .046 with the Best gaskets. Ted Eaton has said that he hasn't experienced any difference in performance with a quench up to .060. 

I have a 292 block that looks clean. I do not know if it has milled at any time in the past. What is the limit for milling the head surface and where should it be measured to determine if it is within limits?