Operating angle for pinion and transfer case.

[Eddie Greenlee]

Morning
So I know my pinion angle and drive shaft angle are about 1deg apart (double cardan). Drive shaft 18.1 , pinion 17.3
But what’s my operating angle about ?
Transfer case yoke 6.1down of course.
Pinion flange 17.3
The difference is 11.2. Deg.
What is the reasoning to knowing this operating angle ?
Thanks