Using my 35mm manual focus kit, I could focus on subject A, check the distance against the dof scale on the lens then focus on subject B, check the distance against the dof scale on the lens then rotate the focus to midway beween the two and select an aperture corresponding to the distances - as listed on the dof scale. I could take the picture and know it was right.

How do I do this with these AF only cameras such as the XT?