Wayne uses Competition Engineering's method for measuring DH's for high domed pistons w/o this 'lip'. Look familiar
http://competitioneng.com/WaltsPage.htm
I think the key for success with this technique is the get the solder end flat/flush against the C wall. The DH will be the flattest part at the C end of the solder, the remaining will be the P to Head clearance .....I just could not get it to work right for me. Better a direct measurement anyway. Yes, DH is the measurement from the P lip to the top of the C at TDC.
The CR's mentioned below are correct for the different SC years. Remember that Porsche's printed CR specs of 9.3:1 are actually measured around 8.9:1. IF you know this engine has not been apart or has not been modified, you could work backwards with the 8.9:1 CR to find your P volume.
As long as your DH is not less than 1mm and consistent on all 6 cylinders, I can't think of anything else........
BTW: are you sealing everything up as you go and hoping your P to Valve clearances are OK?