With Vandersteens, its from the front of the cabinet...others may be different. I advise also to use a laser and mark the back wall with an X or dot for each of the speakers and shim one of them so that the dots superimpose on each other.
Measuring Speaker Distance
When speaker placement is recommended to be, say 3-5 from the front wall, is that measured from the back of the cabinet to the wall or from the front driver side to the wall?