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with a modern BMW, it's really not possible to charge a system by pressure any more. As one knows, pressure varies by temperature and therefore just charging to a pressure (either the low side or high side) just isn't accurate enough. The issue is compounded on the e90: The tolerance band around a full charge (1.1 pounds) is less than an ounce. In my opinion there just isn't enough 'slop' for a DIYer to add refrigerant to the system. Even if one were to start with a system that had been properly evacuated, it's really hard to calculate how much refrigerant is in the system versus left in the can, the manifold gauge and the lines.
I used to think you might be able to charge an automotive air conditioning system like a home A/C unit: determine the required superheat or subcooling, record the appropriate dry and wet bulb temperatures and charge to a calculated pressure based on the refrigerant type. However, since I don't know that the superheat or subcooling is for an automotive air conditioner, I can't follow the same practice. Likewise, because all cars of a given model/year have the same A/C configuration, the system volume is known, one simply needs to charge to a given volume.
I don't mean to totally discourage folks from charging their own A/C unit. I've done many cars myself with satisfactory results. My point is to alert folks that the e90 (and probably other BMWs of the same vintage) have relatively low charges of refrigerant and very tight tolerances around what is considered full. This makes overcharging a definite risk.
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