Get BMW coolant and mix some up in a container with 1/2 distilled water. Then you have a 50/50 mix you can pour directly into the expansion tank.
Often you will not have a low coolant light until the coolant is really low.
Make sure you fill a bit short to the full mark on the float, BMW recommends the coolant level be set at 68F, if you set the level say at 40F to full, this leaves less expansion room.
Replace the O-ring on the temp sensor in the lower radiator hose, these leak all the time after 5 years and never leave coolant on the ground. O-ring part number is the last item in my signature.