Calibration volume depends on your speakers efficiency and power going to them. I have found that you should try and make the ms-8 sweeps around 50db-55db for good results. Best way to do this is play the pink noise through the test menu, adjust gains so all channels are equal, then adjust the ms-8 volume until the pink noise is around 50-55db. Then calibrate at that ms-8 volume. But by all means have a play with the volume and you'll find out what sounds best for you.
Also change your front set up to 50hz@24db for underseats and 200hz@24db for the HO.
2008 E92 335D SE - Ohlins
2010 E90 320D SE
1994 E36 318I SE
1996 E36 316I SE