750kbps is the IP profile you'd expect at your previous connection speed (1050). You're current speed of 1376 should get you a profile of 1000kbps, given time. But you probably already know all that by now.
I can't see that .doc properly as I'm trying to render it on a Mac at the moment, which does display .docs, but the text is garbled. But among the garbled text I can see it does seem to indicate your 'new' target margin is 12dB.
I'm still mystified as to why your previous snapshot seemed to show a connection at a margin of initially 19dB. Additionally, a margin reduction usually equates to a speed improvement of something like 600kbps (though it can vary a lot) for each 3dB. Your margin dropped from 19 to 12, with only about 300kbps improvement which, even allowing for variances in the maths, also seems odd. Maybe the router had got a bit confused and the power-off fixed it, but I've not heard of such a thing happening before. I'd ignore it, if it were me, unless it happens again.
BT's DLM aims for a 'best case' margin of 6dB, and that's what everybody gets when they first take the service, but DLM then adjusts the margin to suit line characteristics - error rates, losses of sync, etc. You are on a long line and I think you'll find that 12dB is not unusual under the circumstances, and doesn't really indicate any major problems - other than the problem of line length. In theory, DLM may reduce the margin in future in steps of 3dB, if your line proves stable enough, but that can take an awfully long time and may never happen at all so don't hold your breath.
If you want to try and 'do something' you could follow the guidance on the website for testing from the master socket with extensions disconnected. If that makes a big difference, you can then try and improve your wiring - disconnecting the 'ring wire' is the usual thing that helps the most. It can make quite a dramatic difference so is worth a try.
