How Level is Level?????

I’m going to service my ’06 Jetta DSG and I want to try the drain and measure method. However, I’ve looked and can’t find how to make sure the vehicle is “level”. I noticed on a utube video that they put a small level on the glass of the sunroof. Nice idea but my sunroof glass is curved.
I looked in the Bentley manual and it says “level the vehicle” with no specific location to put a level. I know that on some tail wheel aircraft we use the cabin door sill to level the aircraft for weight and balance checks.
If nothing else I guess I’ll use the frame rail that has the jack pads as a starting point. What does everyone else do?
Thanks, Steve.

Try your level on the door sill. It is really not that critical. I think the idea is that you should not drive the front wheels up on ramps. If your surface is curved then stick a like sized shim under each end of your level. so it won't rock.

I have encountered vehicles that had less than 3 liters of DSG fluid without ill effects in the way it ran. If the fluid pickup is not sucking air the car should move.
Too little fluid will just work what fluid is in there harder and hotter.

Great. I checked my frame rail between the jack points and then checked the front door sill. Both were level and from my calibrated eye ball, it looked level.
Thanks for the extra advise.
Steve.