I too tried bifocals, spent an arm and a leg getting them to what I thought and read was right. Guess where they are, in the same box as all my other great ideas. Normal prescription glasses that are set to the front sight will always come out in front. Your head only has to be a fraction off and you are not looking through the correct part of the bifocal lens. When you are cold, hot, tired or just plain sore you hold your head in a different place, or hold your shoulders.
If you do want to be bold, I had my centre of the glasses ground slightly higher to compensate for the prone position, it helped me.