OP asked where to put clay to measure for deck height. That’s not how you calculate it. Deck height is simple.
Porsche cases are 112.4from crank center to cylinder mount surface. Cylinders are 85.4 totaling 197.8. That is standard Measure measure measure
If you are trying to figure piston to valve clearance that is totally different. Put play dough across valve relief in piston fully assemble engine rotate engine at least two full revolutions . Disassemble and check clay thickness that is valve to piston clearance PITA on these engines.
Now if you have bored the cylinders like 95 open to 98 and you have not opened the cylinder head combustion chamber to match then measuring with clay like the above picture. ok. But that is not deck height. That is just poor work and no attention to detail
When building an engine any engine takes planning That’s why piston manufacturers ask what cam lift and duration want size valves combustion chamber timing etc. that’s why you have to pay close attention to stroke rod length piston compression height. That way if you have to add a base gasket you know how thick.
I feel the only thing putting clay in the combustion chamber will show is poor planning or poor execution. Take some veneer calipers and measure you cylinders the measure the combustion chamber should be the same if not fix it
Deck height is simple