Average angle down (looking from the side of the engine/trans combo with the trans to the right) is usually in the 3-4 degree angle. Center-line of crank thru the center-line of the trans mainshaft. At that angle the carb base of the intake manifold will be level from front to rear and should also be level from side to side.
The average angle up for a street driven vehicle of the pinion shaft center-line of the rear end should also be in the 3-4 degree range.
The two angles should be parallel to each other looking from the side and looking from above. They do not have to be inline with each other but must be parallel.
A problem could come about if you have a very short driveshaft and/or the engine/trans is mounted very high. This would cause a steep u-joint angle even when the engine and pinion shaft are set at the correct 3-4 degrees and are parallel.
The 'fix' is to lower the engine mounting position to decrease the driveshaft angle. The third member is pretty much at a fixed height because of overall tire diameter. Changing the tire diameter can alter the height of the pinion shaft but it would be more correct to change the engine height. Keep in mind that the 3-4 degree down angle of the engine must still be maintained.
There are a few exceptions to the engine angle. They pertain to boats and drag race only vehicles primarily. The above info pertains to street driven vehicles.