I'm fairly new, as well. I have gone through this exact thing, though. Assuming the bank is straight and there is a parallel line to the bank where you are standing and that line is the 0 degree point, and you are casting across at a 90 degree angle, you want your anchor point, in general, to be about between 5-25 degrees off the parallel line and one rod length away from you. Here is a little quote from Jeff Putnam:
"The presentation and overall execution of a spey cast relies heavily on the formation of the “D” loop. This is the most difficult element; however, it is the most important. Let’s take a look at a few key points of information in order to understand how to create a better “D” loop. 1) The Anchor Point Placement, essentially the bottom of the “D” loop that makes contact with water, should land slightly in front of a 90 degree angle, one rods length off to your side. This location is optimum for loading; however, it can be placed accordingly based on the amount of room needed behind you to form the “D” loop. 2) Just as important as the specific location where the anchor point is placed, is the manner in which it lands on the water. Once its lands it must stop immediately. It should not touch down then bounce, skip or skate across the surface. 3) The load for the forward stroke is based upon the tension in the “D” loop that is created from a solid anchor point. If the anchor point is moving or not laying on the water or too far under the surface the rod will not properly load. Another important element of the anchor point is that it should be very straight and aiming toward the target that you’re casting to. The “D” loop should be formed 180 degrees opposite of the forward target."
You should try to get consistent with the placement of your anchor point in relation to you, but as a newbie, that won't happen right away, which is good and bad. It's good because you will learn to adapt your stroke to account for different placements, but it's bad because you can't control the anchor placement well. Once you get better, you will purposefully change your anchor point placement depending on conditions and obstacles that you need to avoid in order to cast. So, this practice will sort of help in the long run, but will be frustrating at first because you will have casting failures as a result of it.
The reason you are getting so much more power when your anchor point is behind you is because you are not loading the rod properly when the anchor point placement is "correct." Probably, you are pausing too much when you transition from your "sweep" and the rod is losing its load, but that's hard to say for sure without watching. But, in some way, you aren't loading the rod properly/fully or you are losing the load somewhere in the transition from sweep to forward stroke. When you have the anchor point behind you, you are using the momentum from throwing the anchor point backwards to load the road, much like when you back cast with a single hand rod. Or, because the anchor is behind you, your rod tip might be too far back, allowing for a longer forward stroke which is loading the rod more from the tension of the anchor point. It's difficult to analyze without seeing the broke, but these are the problems I had when I first started, and still have sometimes when I'm fatigued.
Take this all with a grain of salt because I am a newbie, also. But, this is what I have found happened to me. Allow others to chime in because I probably said something incorrect, but that Jeff Putnam article is pretty good and he's a really good instructor.
Hope that helps a little.