I think that would work if you are working with an analog mixer. If you are going for a digital mixer(yuk!), I would go for - 3.0db for insturments -6.0 for drums and sub bass (below 250k). Now if you are doing any kind of percusion heavy music like Drum and Bass/IDM/Breaks/Prog House then do the opposite of what I just talked about. This will give you lots of head room for the main instuments (or percussion) that you will need without that nasty digital distortion. Industrail music I would toss all of the "rules" out the door. anything that can make the shit grity. I hope that this helps. Also if you are using a software only setup then alwaysuse the digital rules.
well you need to give us more info like ANalog synth into Analog Mixer.....or Digital Synth iunto my DJ mixer etc......there is different headroom for different gear. try xxxxxxmaze on aim I might be able to explain it to you better. I work 10:30-7:30 California time