I would recommend an adjustable front sight so you can experiment with diameters. As Howard mentioned, it is more common to have a larger front dia. but it's not for everyone. For myself I find it much easier to have just the smallest amount of white around the bull, otherwise I just keep looking to see if I still think the bull is centered in that mass of white. I even like no more than a couple of inches of white around the bull for offhand. I have experimented with this with my air rifle and found that when I shoot with a very tight front apperature my groups are about half the size. While this works for me, it's not for everybody, that's why I would recommend an adjustable front you can find what works best for you.