Trout - for once I disagree with you, as do many northern countries.
I find around 6% towball downthrust perfect for my D4 pulling the 3.5T brick, and it was the same with the D2a (albeit a bit slower up the hills) - but much more relaxed driving. HOWEVER, I always made sure that I also had a slight lower trailer front end than the rear.
Don't claim to be as expert as some on here, but 50 years of heavy vehicle driving might count for something.

