Volume for final acidic soup from 1 kg MHRB?

[potato]

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.

 

[Gwn]

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.