spider66

Is my C heads mill?

don71

Try measuring from lower bolt hole boss instead.

2.2XX plus or minus average.

OLDSter Ralph

The #1 way is to CC the combustion chambers. This measurement could be a general "estimate", because it determine rocker arm stand height. Without looking at the manufacturing blueprint, this would be held to a closer tolerance than the 2.2XX headbolt boss thickness.
.....Just my two cents worth.