I've been shooting Bullseye competitions for about 30 years. The NRA Conventional Pistol targets are intended to be scaled so that the scoring rings subtend the same MOA, regardless of distance. In practice, I and most of my fellow shooters find we do just a bit better on 25-yd targets outdoors in the summer than we do on the 50-ft targets indoors over the winter, and if you do the math it turns out the rings on the 25-yd targets are just a tiny bit proportionally larger than on the 50-ft targets.
That aside, assuming you're talking about the Slow Fire (B-16) target, in competition you would fire 10 shots in a period of 10 minutes, which obviously allows you to put the gun down and rest between shots (and you should). With regard to your question about what's a good score, having seen literally hundreds of Bullseye shooters and many thousands of Bullseye targets come and go over the years, I tend to think of a pretty good target as one that has all shots in the black (as Krell1 mentioned). My experience has been that such a target tends to score in the mid-80s or higher (a shot that just barely touches a scoring ring is awarded the higher score, so a shot can be almost entirely in the white "6"' ring yet score as a "7", which is the first ring in the black). Scoring in the 90s in the Slow Fire stage is very good shooting indeed, and only the top competitors in our league do that consistently.
Maybe with two hands at a rest with the gun in a vise. 
