No,No No.
Don't mess with the Tranny, and the nose of the diff should always point down to the ground to keep oil in the bearing.The front angle must be like this ^,driveshaft going down relative to the tranny.
And the rear must be like this V, driveshaft going up relative to the diff.
The car being level matters not a bit.
But if you raise the car, the jackstands have to support the axle tubes and the car should then be raised at both ends to maintain the same weight over the rear springs, cuz if the spring arch changes, so will the pinion angle.
So then it's best to have the raised-up car sitting with the same body-rake, as it had on the ground. This reading is best done on a 4-post drive-on hoist.
Getting back to the angles;.
If the driveshaft goes down from the tranny at a 5* angle relative to horizontal ground plain, and the tranny pan is 2* down at the back, relative to the horizontal; Then the driveshaft is said to be 3* down; (5 less 2); the difference between the driveshaft and the tranny.
If the driveshaft goes down from the tranny at 2* relative to horizontal, and the tranny is 2*down at the back also, relative to horizontal, then 2 less 2 =0, and it is neutral. The measurement is zero difference between the pan and the driveshaft.
Unless the car is severely lowered,the driveshaft will always go down relative to the pan.Correcting for such a driveshaft-going-up situation would be impossible.
For cancellation of vibration due to the u-joint oscillation, the rear angle should match the front; when loaded.This is not always possible. If the car has a lot of power, enough to wrap up the springs, some amount of additional nose-down will be required to prevent u-joint failure. How much depends on how soft the springs are, and how much torque the power-house is putting down.
So whatever the front angle is, you must have at least that, at the back, and some additional amount to protect the U-joint when under full power.Typically, the rear is set up with 5 to 7 degrees; 2 to 4 for the match-up and the rest for wrap-up.Just remember, this angle is between the driveshaft and the pinion. It's really hard to measure it directly, so we measure the installed D/S relative to the horizontal,then remove the D/S and measure the yoke, also relative to the horizontal. Then a bit of math and it's all over.
If you were to raise the tranny, you would increase the pan to D/S angle,(a bad thing),and then you would have to increase the pinion angle in compensation. This would make a mess of things. But if you were able to lower the tranny far enough to make the pan angle equal to the D/S angle, then you would be able to reduce the pinion angle that much more.
However a zero angle is bad. The u-joint needs to run 1 or 2 degrees to shuttle the grease around, but more importantly to prevent the same bunch of needles from doing all the work, all the time. You want 1 or 2 minimum at the front.
You cannot remove the D/S, and measure the tranny and pinion, subtract, and come up with a number. That number has no meaning. The numbers have to be relative to the driveshaft.
The important thing to remember is each angle has to be measured relative to the driveshaft.The front needs to be a minimum of about 2 degrees,and the rear needs to be more than the front.