My friend has the same problem with the fiber optic sights. He gets what he calls a halo around them making it hard to align. And Frontier's post about having a high contrast point of aim helps too. And IMO, you may want to use a bigger bull - like a 6" with maybe a 3" center.
Hope this helps.
I usually don't worry about shooting at 100 yds. But I do just to see where my bullets hit - just in case. Normally where I hunt, even a 50 yd shot is long. But occasionally I do get the opportunity to reach out there.