After trying out various configurations, with 10kg of front-loaded panniers (problem was too high on my rack and the bike felt like I was riding the SS Titanic on stilts), everything on the back (except for the frame bag) which felt OK, I finally decided to fill up the main triangle to keep the weight between the wheels and the ride was most agreeable. I've estimated the final ratio is
36.5/63.5% - I should add that this figure includes the rider (my own!) weight!
It was the best compromise I could make with the time I had left.
I've updated
Touring Latin America accordingly.