My trick was to buy a Polaroid MP4 reflex viewer. Very easily to adapt to it. Not something you would want to carry around w/ it mounted on the camera, but mine was just for landscapes and portraits where I had the camera on a tripod. It was held on w/ two small bungee cords, but you can file the little nubs and mount them more permanently. Other times I simply focused w/ the rangefinder.
Been painting 50 years and never inverted a painting. What I normally do if there is any doubt about values is take a B&W photo to eliminate the colour. That way, all you see are the values. I have tried looking at a piece in a mirror to ck compositional balance too, but it never works. Often, balanced or not, the way it's composed on the paper is the right way. After a while you just know whether its right or not, and don't need to ck any other way other than looking at it.