The guys at Bimmersport said if the mix it too rich it can cause sludge to build up in the cooling system. The BMW coolant says @ at 50/50 mix its good down to -40*C. You live in brandford, not the yukon.
When I replaced my rad I mixed mine maybe 60/40 (coolant/distilled water)
Make sure you use distilled water, you don't want the minerals and crap from regular tap water building up in your cooling system. (your local grocery store sells distilled water usually $2 for 4L or so) Also, use the BMW coolant. This seemed to be the consensus of everyone I spoke to about it.