Yes, agreed... you should be focusing on the foresight and the target should be out of focus.
Shooting frames can help, even with mild prescriptions, but you need to make sure your optician makes the lens so focusing on the foresight becomes easier, not the target.
To set the rearsight iris, open it up so it's as bright as can be, then turn it down to where you notice the image dim... then stop. Normally that's about 1-1.2. If you have it too open the depth of field is more shallow than it needs to be, and if you have it too closed then the image is too dark, meaning you lose contrast and your eyes can tire because they are trying to open up all the time.
A blind can also really help. I would try one of those before starting on anything expensive. Just a bit of cardboard or opaque plastic milk bottle will do for a test. Helps the aim eye to relax.
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