[Jay Shah]

If you don’t want to use the DoF calculator in the field… you can always roughly focus third way into your scene.

[Jay Shah]

In aperture mode, you will not be able to manually control the shutter speed. You can use exposure compensation, but that will change the ISO first. For this you can use the manual mode, set the aperture to the lowest, set iso to auto(min and max) or desired iso and then manually change the shutter speed.

[Jay Shah]

FYI: if the camera computed these settings in aperture mode, there is a high chance you will under/overexpose the shot if you change the shutter speed in manual mode while keeping aperture and iso the same.