Crossover Point

I'm trying to figure out the crossover point between the Canon MP-E 65mm macro lens and the lowest power on the Apex Practitioner microscope. It's somewhere around the 1mm mark. I've still got a long way to go to get decent focus stacks using the microscope, but this image is my first serious attempt. 

Dicyrtomina saundersi

18 image focus stack, Zerene PMax.