The difference between the car "level" and the ATF pan "level" is minimal (I believe around or below 1°), which will not affect the ATF volume much. If you observe the Bentley for the correct temp, there is still some +/- tolerance. It's not pharmacy.
When i did my ATF change, I was also paranoid about this. Once I had my car on stands, i had a level between the front & rear stands (the leveling benchmark was a string at same height between the said jackstands, and then placed the level on the string - all was OK). Then, I placed one of those magnetic levels on the pan. It was very close to the big level.
That's probably why the TIS mention to "get the car level".