Bridge is placed in the center of the instrument. Use a divider to measure from the medial border of the ff to the lateral border of each bridge foot. Adjust divider and move bridge as necessary to effect equal spacing on each side of the bridge feet. Once the position is established, the lateral margins can be marked by placing small pieces of drafting tape on the top of the instrument.
The bridge foot should "overhang" the bass bar by .75-1.5mm. (This is measured using the two-pronged, metal bass bar/soundpost gauge.)
On an instrument that is already strung up, once you've verified with a divider (measuring from the medial border of the ff to the lateral border of the bridge foot) that the bridge needs to be moved laterally, that movement can be accomplished by tapping the bridge with a dowel and a small hammer.