From 911 FWSM Volume 1 page E78 circa 1965 for a 2.0L
(C) Porsche AG
I believe this dimension was retained through all models to 1989 - others might verify or offer alternate info.
If you beleive the heads have been cut - consider the top surface of the head as your datum....(less frequently machined ex factory)
Can also confirm 27mm total dome depth that Tom measured above (taken from a 3.3T head)
John