Ian
A good practice technique to aid concentrationon on the foresight is to shoot on a blank card, I often do my sighters like that before a match card.
I also use shooting glasses with the prescription lens set to the focal length of the foresight, so I can't focus on the target if I want to, sadly that does'nt stop me trying, so It still takes practice to just focus on the foresight.
Best regards and good shooting
Robin