Simple really, put some news print on the wall and sit the correct distance from it to get it in focus and then rotate the lens in your fingers until you get the best picture and the least distortion. Then have a look at where the notch is and use that to help you put it in the holder, my guess the notch is at the zero meridian to allow the optician put it in the frame at the correct angle but the way to do it above works too and you will be surprised how often lenses get put in glasses incorrectly at the slightly wrong angle. You may even be able to rotate the lens while it is in the holder which makes it even simpler to do. If you cannot suss out the best place using print draw some circles and use them and when they are the roundest and not distorted you have found your point.
On another note regarding your iris and the use of it, don't use it to help you focus and increase the depth of field eg. bringing the target, front sight and back sight into focus, you are better without it at all, you really do not want to see the target that should be a fuzzy blob, the back sight should just be out of focus and you should be able to see the molecular structure of the front sight as clear as day. Why do some people use them? at different ranges there are different ambient light levels and they use them to get a controlled amount of light going into their eye and not to help focus, the aperture used is large, leave it in the box, your balance will be better too.