Hope these are of some use

https://www.wikiwand.com/en/Diopter_sight
https://www.quora.com/What-scopes-ar...iathlon-rifles
https://bedstargetsupplies.co.uk/wp-...is_Fordham.pdf

If the gap between the target black & front sight ring is to big you may well want a smaller front sight element (or an adjustable one).

lf its too fine a gap you'll be edging the target black into the ring of the foresight & loosing accuracy/grouping so you'll need larger gap

As has been said a good book on the subject will help