I was surprised to find no mention of this subject in
alt.building.construction or free.uk.diy.home and nothing detailing
how to go about it in uk.d-i-y.

I am about to put some up on a rather large building with the
downspouts at the corners. I was wondering what would look the best
just to put the brackets up and start fitting or to mark out where all
the lengths are going to end in order to get some sort of symmetry.