Best location to measure for true level

I'm gonna be replacing the transmission fuuid on my 5.7
Tundra. As you know the truck should be as level as possible.
On my (25° pitch aprox*) driveway
Using 4 jackstands it's 2wd Sport and I need work room.
Is the frame underneath the doors the best place to use a med size carpenters level to get it as level as possible ?? Or is there a easier / better way ??
This may sound anal but there are a few factors at play here.
1. It's my first time doing the extraction via the transmission cooler line
2. I'm doing my best to follow instructions.
3. I can not Fudge this up. It would set me back $$ major league. Plus I Wana make it easy on me...and not do shit twice...
4. I say 25. it may be closer to 20 or 18° of pitch. Our side of the St is consistant...across the St it's more like 25-30° no joke.
Any help Appreciated.