I don't think you'll ever achieve the same diagonal lay with the Clark; it's not really designed for it.
But . . . once you get the sag right, the Clark is incredibly comfortable for back sleeping. I like the "banana" position when I'm on my back.
And as others have stated, try deflating your pad a bit.
What I do is blow it up all the way. Then I get in and get to a good position. And then I reach back and unscrew the pad nozzle to let some air out slowly. I just let it start to deflate with me on it until it feels good and comfy . . . then I screw the nozzle shut.