Here is my advice.. Welcome to the forum btw...
Forget about the green button, forget about TAv Shift. Go into your custom menu and set M mode so that the Rear E dial controls the shutter speed. Set your Preview option for Optical preview (pg 129). Set your camera to M mode. Select your Aperture on the lens (we are talking about Non Auto Aperture M, K, M42 type lenses, correct?), and while looking through the viewfinder, Pull the power switch to the preview position. Your Meter will appear (get rid of that horizon level too). With your thumb, adjust the shutter speed on the rear dial until your meter is either centered, or just to the left of center (which is where I find it to be optimal). Make sure you're focused, and shoot. Alternatively, Select your shutter speed and use the aperture ring to set the exposure. Pulling the preview switch stops the lens down so you can see what you are really getting with respect to DOF and exposure. The green button does the same thing but only momentarily. It's a little cumbersome at first but once you get the hang of it, second nature. There is no need to use the DOF preview with M42 lenses because the camera does not control the aperture.
Others will disagree but that is what works, Reliably, for me.