Sit on the bike (easier - get someone around your weight to sit on the bike) so that the S/A is about where it will be when ridden.
Now tighten the nut until it clamps the S/A good and tight, do NOT back it off, the S/A rotates as much as it needs to by the rubber in the silent block bushes flexing, this is on the face of it pretty unbelievable but it is how it works, the inner tube of the bush is clamped firmly between the frame sides and the rubber takes the strain.
If it's not tight all you will accomplish is to wear the end of the inner bush tubes or (worse) the frame