The whole crown of the piston.
Delta T is a factor. How much the piston material absorbs is another as is how much area is exposed at a given time.
And on this whole subject against popular belief I personal found that not squeezing the quench area seemed to make it more manageable also as my first motor was 15.25:1 comp but I put the pistons are zero deck height with grade C gaskets. This allowed me to run more timing without causing any of the normal issues associated with higher timing levels and I was extremely happy with that motors response. There is a LOT to the complete combo but this is just a nice example of what I did back in the day with mine. I really do miss that motor cause it was one of my favorites of all time!