Steff,
rule of thumb for setting angle of rear shock / spring.
As you already know - a triangle is the strongest [ 2D structure ] form of tubing layout. The same applies if you replace one of the elements with a shock [ raving generalisation ].
However the theory obtains. Sit you butt on the bike and adopt the most normal posture - get someone to take a line between your hip joints [ side to side ] - then take that datun and strike a line to the rear lower mount [ proposed ] for the shock [ Twinshox here ]. That will give you a rough vector for the line of force. This is more important for a longer swingarm element.
Just a Christmas thought amongst the Turkey and Brandy ............................