I meant clamp alongside, then screw it to the old/existing 2x10 joist. The only concern I see with that is some potential tortional load on the joist. But once screwed to the subfloor that should go away; locking the position and preventing any twisting. For a shim, I like your idea of scribing. Still, will take a very accurate cut and it's not necessarily a straight line.. I could get 2 out of a 2x6 no problem. Maybe more. Save a little $. But more room for some error/thick/thin in places. If I'm going to all this trouble, might as well be dead flat. Still, may be worth a try to see how that goes. But your input gave me an idea... If I use a board to clamp to the side as the guide, it would be a pretty good reference to then add shims. Make shimming easier anyway. Fine tune with thin material as necessary. But shim would need to be something that would not blow up when a screw hit it. Still, just adding the boards is the easy button, I think.