As a general rule cut the seat about 40% to 50% into the prong for
harder tougher stones. diamonds and the corundum's. And, up to 60%
for lesser hard or tough stones such as emeralds.
I thought the rule regarding notching prongs is to never cut into
the prong more than one third (ie no more than 33%) of its diameter.
I presume this is for strength and to avoid prong failure.