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There could be a big difference between 1 inch from the top and 6 inches. Undoubtedly there is an expansion tank or chamber somewhere since hot liquid expands in volume. If you really did fill it almost full, it would just come boiling out the relief valve in the cap or blow the cap off if it doesn't work.
My 6" measurement is based on the metric info I got out of TIS.
The warning message is a helpful message but also liability mitagator. Yes, if you open it while it is hot you can get badly burned. When you try to sue BMW to cover your medical bills, their lawyers will pose that you were warned by the car and you did not heed the warning, so BMW is not responsible. Liability mitagators are to be seen constantly here in the USA, where most of the world's lawyers live and work, and most of these type of liability lawsuits are filed.
Finally, general advice to everyone - don't be cheap or lazy about fixing cooling system problems. This is a $100k USD car, it has at least 7 radiators in two cooling circuits to cool the engine, turbos, DME ECUs and so on. This is why there are no fog lights on the F10 M5. That space is needed for airflow through radiators that other cars don't have. If you get a bad failure of a hose, pipe or connection, or even a leak in a radiator, you can start losing coolant fast enough that you not be able to drive the car. Maybe even damaged something badly. And not that much of the cooling system is visible without a lot of disassembly, so you won't just fix it by the side of the road or at home.
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