What eye deviation do you have? + or -? And how much?
I have considerable eye deviation of about +2.50, though I can focus fine on front sight my eyes only tire relative quickly.
Allthough eyesight can get worse over time, I doubt focuspoint can shift about 10 meters. Cause thats what yr talking about.(from front sight clear to target clear).
Rear sight should be blurred. You cant focus on 3 things at a time.
Focus should be at front sight.
An often made mistake is switching focus between sight to target and back.
Allthough you may get away with it when young, you cant when older because eyes are less flexible and cant focus that quickly anymore.