What is a good volume for the final reduced acidic soup produced from boiling 1 kg MHRB?
I currently have about 3.5 L, reduced from 15 L.
During the final reduction, a film rapidly and repeatedly formed on the surface of the soup. I have heard that this indicates a suitably concentrated soup, but 3.5 L feels a little large.

I'm no expert potato, but from my limited extractions I've found too loose is better than too thick. Sure is more extraction vessels, but my yields have always been better when I didn't turn it into a thick sludge.