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".
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39, e53, e46 & Tribby