Carrier Arm Set-Up

[Harry]

The BBB instructs to set the distance from the pinion flange face to the upright arm as 158 +/- 1 mm.  It also says to set at 90 degrees to the left axle.  So my question is the measuring point on the arm for the 158 setting.  The Carrier arm is a curved surface, I.e. there doesn’t seem to be a straight/flat surface?  What/where is this measurement made from?
I guess I’ll use a level and right angle to set the 90 degree dimension.  Advice?

Thank you,
Harry

[dirkbalter]

Harry,
For what it's worth, I measured  the 158 dim at the end of the radius in the upper area, close to the stud. If I remember correct, it becomes somewhat flat or straight.
On the 90 degrees, after setting the axle tube perfectly horizontal, I also used a level in conjunction with a square, for the angle. 

[Harry]

Thanks again Dirk!