I made my own loupe out of PVC pipe and part of an old camera lens. Specifically, the front element of the 45mm Rokkor lens of an old Minolta Hi-Matic camera works just great.
I agree with Sandy. I use a small magnifying glass to get the final focus and to check points across the ground glass. For the initial focus, I pull my head back far enough so that I see the entire image and check to see if If I have the desired compostion and aim point.
No loupe for composition, loupe for focusing.
But I shot mainly still life, I think that for outdoor sessions (meaning landscapes) loupe it is not so important.