The question does not regard what tool to use to measure "level". The question is what or where do you measure to determine a "level vehicle", as BMW recommends.
Is it the pan, like ECSTuning recommends, which tilts the car forward and allows more ATF to be put in before it dribbles out?
Or is it some other part of the car? Such as the door sill or rocker panel.
This question is posed because there is a significant difference between these two DIY procedures which will significantly alter the amount of ATF that can be put into the pan. The ZF tranny is definitely sensitive to ATF amount as my experience is that as little as 1/2 qt difference will alter your shift points. If the ECS info is incorrect, I would hate to see someone damage their tranny using misinformation. This is not an inexpensive part to replace...