Only mandatory requirement is that the upper section be inserted at least below the "min" hash marks. After that, it is a matter of comfort. Dahon has afforded us a maximum amount of flexibility by giving several degrees of freedom.
I am carrying a pen and marking positions that seem comfortable. I'm made two changes already in the angle of the handlebars to allow me to change my hand-brake relationship, and have already begun to drop the stem height a little bit.