generally- if you're going to send the be sending the final mix out for mastering, most mastering engineers/houses want somewhere between -18dB and -10dB so they have enough headroom to work
If you're mixing/mastering your own tunes- -2db seems a good level to try and NOT EXCEED- digital clipping/distortion is much uglier than analog, in fact that is why tape style has an appeal to great many artists