Here is another suggestion for determining "level" based on ride height.
Ride height is measured from the lower edge of the fender arch to the bottom of the wheel lip. The key is to take the difference. A level car will have the fender arch front and rear differ from each other by the difference in ride height spec. For the purpose of checking transmission fluid level the absolute height is irrelevant; the higher the better for convenient access to the transmission. For instance, with my car the specs are 592 mm front and 560 mm rear; a difference of 32 mm. When the car is sitting with the front fender arch 32 mm higher than the rear arch, the car is "level." Side to side at each end should be identical of course.
In spite of what you eye may tell you, your garage floor is probably neither flat nor level. For instance, most garage floors have a drainage slope of ~ 1/4" per foot toward the door. Over an E39 wheelbase this works out to 2.3 inches or 59 mm difference in height. Similarly, side to side level isn't necessarily so level either, or flat. You can easily check. Pour a pail of water on your garage floor and watch it run/drain toward the door and likely puddle in some spots. Draining => out of level. Puddling => not flat.
An easy and accurate method to check relative heights both end to end and side to side is a water level. All it takes is a ruler, a length of clear tubing, a few cups of water and perhaps a funnel.
FWIW, I wouldn't worry about being "out of level" by +/- 10mm or so. It's within TIS specs for ride height differences and works out to 0.20 deg front to rear, 0.38 deg side to side. One likely has more difference from "level" due to relative settling in the engine and transmission mounts. It just won't make any practical difference in the fill amount.