The range feature is an estimation based on a moving average of your fuel economy over the previous driving interval (seems to me that it is about 10 miles or so, but don't take that as gospel). Therefore, unless your present, instantanous consumption matches this average, the range indication will be of limited use.
HOWEVER, the average fuel economy (i.e. mileage) is a calculation for which the OBC can and should be very accurate -- down to the tenths of MPG. Mine is way off (says I'm getting an average of 32 MPG for a tank of gas, where I'm actually getting 23), and I keep getting the run-around at the dealer about calibrating it. Since the range calculations depend on the MPG calculation, my range feature is completely useless.
In MY99 and 00 E46 models consumption calculations could be adjusted through the instrument cluster test mode. This "test" entry (I think it is test #20) is no longer functional in MY01 and beyond. There must be some other way to adjust this, perhaps through the MODIC, to get this accurate.
If anyone has more info on this for late model E46's, please post!
Dave