YOU NEGLECTED THE TRANSMISSION SHAFT ANGLE
If you happened to jack the car up so the transmission shaft ANGLE is NOT LEVEL with ground, your instructions will be INCORRECT
(What would you do if this was some wild eye'd "gasser" all jacked up?)
Therefore you NEGLECTED the transmission shaft angle.
This is called "that's how driveshafts work"
The ANGLE is that the two angles of the two u joints MUST CANCEL that is THEY MUST BE THE SAME
The reason for this is that the shaft does NOT turn a "constant speed." The driveshaft actually speeds up and slows down during it's rotation AND THIS IS CAUSED by the angle coming out of the transmission shaft.
The only way to mitigate this CHANGE IN VELOCITY of the driveshaft is to CANCEL it back at the rear axle by having the second angle exactly the same as the first angle.
If you do not know what the first angle is, then you cannot set the second angle.
It's also why, on some 4x4's etc, that a so called "constant velocity" joint is used.