(Why did I let myself get drawn into this mess?)
Motor/transmission center line in relation to pinion center line = proper way to measure. Not too difficult, some don't get it. Some will argue till the moon turns to swiss cheese. physics says this is the proper way to cancel change in speed due to an angled joint. Run the joints ninety degrees to each other, all is good if the trans and diff are close to parallel. Some angle is needed to properly keep the joints lubed and keep the needles from flattening due to being at the same spot all the time. If the engine/trans measurement didn't matter, then the diff angle wouldn't either. Run one too far out of alignment, you get broken trans parts. I will try and load a photo.
If the trans is pointing down and the diff is pointing down, add them. If either is going a different direction, subtract the smaller angle from the larger. If both are pointing up, fix em.