Update: I think I'm starting to get the eye relief thing under control for the RPR. I will, though, have to resolve the astigmatism issue. For exmaple, when I use my right eye (which I would, normally) to sight in, and I get a good reticle picture, and I then switch to the left eye, the line of sight changes totally. It would appear to be off by the diopter of the astigmatism. So we'll see what needs to happen.
As for the AR-15, I'm thinking of getting a "dimpling jig" to help align the gas port and block....
Are these things worth it? It's only $40.00 but if it helps align the port and block, it's well worth it. Then, all I have to worry about is the tube.