IMO, there's always a trade-off with placement of the VSM. As I've posted many times, you get some welcome room reinforcement in the bass when you get close to the walls, but imaging is optimized in (as close as you can get to) free space. If you've found a point vis a vis the side walls where imaging suffers when you get any close, you could try scooting the speakers a few inches close to the wall behind them.
As Mapman notes, it's not really possible to cite specific measurements, since that will be both room dependent and - to a real degree - a matter of taste.
As Mapman notes, it's not really possible to cite specific measurements, since that will be both room dependent and - to a real degree - a matter of taste.