Last week, I installed a JPoint (4 MOA Dot) on my FA97 .357 Mag. I also had to install the shim (large end forward) just like TommyD. I didn't have any problem adjusting the POI -- just like a Bow, or moving a front sight -- move the dot to the point of impact. Anyway, a couple of observations.
First, depending upon light conditions and which glasses I was wearing, I was getting some dot flare, which impacted accuracy some. When I could get a "clean" dot, results were very good. I normally wear a pair of 1.25x diopter safety glasses when shooting open sights. The 1.25 gives me a clear front sight without blurring out things at a distance. When bench testing loads, I'll go to my reading glasses (1.75x diopter) under my Zeiss shooting glasses for a "crisper" front sight picture with somewhat blurrier distance objects. Either magnification caused the Red Dot to blur or flare. The clearest red dot came while wearing the uncorrected Zeiss shooting glasses. When the dot is clear, it is very precise.
Most of the shooting I was doing was with the sun behind me. There were some scattered clouds. I'd get dot flare with the sun behind me, and the dot would clear up when a cloud passed in front of the sun.
The "flare" for all practical purposes just makes the dot larger (less precise).
Just like shooting a scope, the JPoint will magnify your shakes.
Here is a picture of 10 shot group I fired at 50 yards the other day. Conditions were terrible. The wind was gusting to about 30 MPH. I was using a blue jean leg stuffed with kitty litter and sewn up a both ends as a rest. As mentioned above, the sun was directly behind me. I was getting some dot flare occasionally. I was getting tired, and my concentration was waning.
You can see 7 shots in 1 5/8" in the lower right quadrant of the black square in the middle of the target. You can also see parallel lines going down to 3 other shots that open the 10 shot group to 3 5/8". I'm sure that the 3 lower rounds were my fault as the 357 load I was shooting is real consistent in the FA.
Paul