Forever the equal but opposite theory was the common theory that was regurgitated by magazines, driveshaft manufacturers, etc. and for most street rod swaps it worked. 3 degrees was the magic number, if you set everything there it would work and for the most part it did. But these aren't 40 Fords and the math doesn't work out.
The reality is that the u joint does not care about the angles on your pinion or your tail shaft and it especially does not care if they are parallel to each other.
All the U joint cares about is the angle that you are asking to work at.....and it wants to work at the same angle as the other U Joint. The lower the working angle, the happier the ujoint will be.
Incase embed doesn't work.
https://www.youtube.com/watch?v=gmV4qwLfOMY