Anchor must be on the same side as the d-loop and downwind for safety of course, and placed forward of the caster in almost all cases. The best advice for spacing as Etacata said, is a rod length out and to the side for all intents and purposes. One could leave it at that...
but since this is the Speypages
An exception is that while switch casting for practice and distance it's not uncommon for the exaggerated d-loop to draw the anchor backward further than a fishing cast would.
There is a lot more than meets the eye here because line taper, length, casting style, and rod characteristics all affect the way the loop generates power (what I loosely term as "compression") and subsequently release off the water but in simplest terms the best place to put the anchor is at the spot where it (a) maximizes the compression of the power stroke coming forward and then (b) minimizes drag after the rod recoils and the line enters flight.
If you change the line design, the anchor placement characteristics will change - for instance a Skagit line verses Simon's new Grandspey. The way you cast, even the cast you choose will make a difference. For instance the Snake Roll and the Double Spey have very very different anchoring styles albeit they are used in exactly the same fishing situation - etc.
For most rod/line/cast configurations it's about a rod length out and to the side, on the same side as the d-loop and downwind. I'll bet that if you interviewed Scott MacKenzie he would confirm the importance of the anchor, but he would cite it as just one element of many that create great distance.
Good luck on your journey.
