If it has a block drain, removing that will greatly aid in getting out more of the old coolant. Otherwise, removing the lower connection of the lower radiator hose will also usually release more coolant. Other than that, you can then leave the drain open at the bottom and continue adding water (garden hose is convenient) to flush out the old coolant. You might want to do a couple fill/bleed/drain cycles also. You should have it all but eliminated from the cooling system by this time. You can leave the tap water in there, or if anal, flush it out with deionized water. Regardless, you're going to have a significant volume of water in your system. Hence, begin your final fill by adding the full amount of undiluted BMW coolant to your system. In other words, if your system holds 10.6 qts for example, you would add 5.3 qts. of undiluted coolant. From there, you can finish the final fill/bleed process using more water, so you'll end up with your perfect 50% mixture.

AM.

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E70 2010 3.0 M57 AT Titanium Silver with Black Leather
E53 2004 3.0 M54 AT Toledo Blue with Gray Leather
E83 2008 3.0 N52 AT Silver Gray Metallic with Black Vinyl
E46 1999 2.8 M52TU 5MT Black with Tan Leather 250K+ miles (Sold, but not forgotten)