Switching from percussion to pressure isn't a hard and fast concept. More than anything it depends on your skill with pressure and the hardness of the stone. I routinely stop percussion way earlier than most and many other knappers seem to express surprise at that when knapping with me. If you can undercut most stacks, hinges, and thick spots with antler pressure why risk swinging at it more. That being said, I don't generally go for cracker thin as they don't look as usable to me.