I have a 1998 Z28 with (5.3) 706 heads that were put on by previous owner. He said they were also milled 30 thousands and Cometic gaskets(MLS) were used to install them. Any ideas on a educated guess on the compression ratio it has now. It definitely only runs on 93 octane.

 

I'm new, could be wrong but I'll take a guess.
The heads that came on your '98 Z28 are like 853/241 heads. 66.67 CC chamber size for 10.1 SCR (static compression ratio)
5.3 heads like 706/862 have 61.15 cc chambers and on a LS1 yield 11.1 SCR (if my math is right).
Every 0.007 milled raises SCR 0.1 point. If yours are milled 0.030 that's just over 0.4 which brings you up to 11.5 SCR .
The GM MLS head gasket is suppose to be 0.051" thick but some say it's closer to 0.054". The usual Cometic gasket people choose is 0.040 because that yields close to ideal quench (distance from piston to head at TDC) . If that's what you have, add another 0.2 so that would bring your total static compression to 11.7 to 1

Sorry it took a month for you to get an answer, I hope I told you right.