The chart tells me that I should go with "+1" sights for the 21 vs. "+0" for the 17. Why is that? If they are very close in sight radius should there be much difference?. The reason I ask is that I can't locate "+1" sights at any online dealer or from the manufacturer direct. The same sights are available in the "+0". Let me know why I can't use the "+0" and feel free to tell my I'm stupid for asking.