Think about lifted trucks - or heavy trucks etc - they have pinions that might be far below the engine/transmission plane. The operating angle of the driveline is all that you care about. And that's a calculation of the various angles involved. In your case engine/trans slope - driveline slope - pinion slope. You don't want a 0* operating angle. Typically you're looking for 1 to 2* of operating angle.
Here's a video that might seem boring - but it should help you get a picture in your head of what you're trying to do.