Hey, Recruit--I've always felt like 3 shots was all it took for me know how well a round is going to work for me. Although that might be partly because I'm ready to quit after that many (ouch!). LOL. The nice thing about the open sights (at least on my Hastings barrel)is that they tend to stay put. If you dont take the barrel on and off, your gun should stay sighted in year after year. If you do take the barrel off, you'll probably have to spend some time getting it sighted in again. Lots of years I nail the first practice shot and just put the gun away I sight in at 50 yds and thats it since my shots are always in the 20-70 yard range. Works great for me.
Good luck!