Target, Bull Sizes and Colors for Open Sight Springer Shooting ?

I'm in my sixties, have a slight astigmatism but can see distance ok. Open sights on any gun ARE a problem.

I'm having trouble. I can't see the bull of the target clear at all at 25 yards. I've been experimenting with different globe sight inserts and seem to get the best results with the tru glo post sight I was able to fit on my JWR Special 300s. ( I put a pic of the sight recently on Joe's 'Springer Groups' thread. ) BUT with a one inch red colored bull on the apx 8 inch black Shoot and See target that bull literally disappears on me sometimes. I'm kinda just centering on the 8" circle as it is about the total field of view within my globe sight. Obviously accuracy suffers.

Maybe my problem is the black target or the total size of the target? Do I just need a larger maybe different color bull? Or what? Thank you, any help would be much appreciated.
 
Shooting open sighted CMP rifles the black zone of the target is the same width as a standard M1 front sight no matter what range, This makes it fairly straightforward to line the bull with the sides of the sight for a 6 oclock hold even at 400 yards. True glow is probly going to be difficult, the sight should be in focus, target blurry. Our 100 yard target is about 6 inches, 12 inches for 200 yards. Try a plain black dot about 1 1/4 inch at 20 yards.
 
In order to get the most precision from your diopter match sights you should select the minimum diameter diaphragm in the rear to increase your eyesights depth of field so that both the front sight and target will both be in reasonable focus. Then you should choose a front insert with a circular opening that appears just a little larger than your target. You might be better off changing the size of your target. Our brains are best at centering concentric circles. I use solid black targets. At 10m the diameter of my bulls are about 1.25". I print my targets from a web based program: http://printtargets.net/