If you have a spotmeter, then put a lens in front of it and call it a densitometer. If you don't have a spotmeter, then get one.(posted 9021 days ago)I don't know if PMK is different (others can advise), but with conventional developers, varying the development time doesn't change the low densities much. Exposure levels control the low densities, and development times control the high densities. Don't base your development times on the low densities.
I don't understand how you can have detail in your highlights, but with 'no separation'.