These days the target rifle fraternity tend to cheat a little bit.
We use 'Eagle Eye' inserts in our foresight, this gives us either a .3 or .5 diopter correction, this improves the foresight/tatget image greatly, the position either in front of the foresight or behind the foresight has also a great effect.
Add this to corrective glasses and you have a big improvement, the other option available is a variable diopter for the rearsight, this is a fixed power variable that allows you to correct for any eyesight defect, big problem is they won't allow you to use both, pity, between corrective lens and these two you could almost have a telescopic sight.