The theory behind the setting of these levels is to ensure that there is plenty of headroom so as to prevent and digital distortion. This headroom is usually 10db.
As Adam has pointed out reference tone should be recorded at the beginning so as to ensure that the next link in the chain can set correct levels. Failing this you must assume that you can lift the signal level by 10db and this will bring the level into line with the other material on the timeline.
This can be achieved by raising the level by 10db either in the timeline or in Soundtrack Pro where you have better control of levels.