I'm really impressed with the JBM ballistics calculator. So far, it's been spot on with my findings. But there's some info provided that I don't understand.

First: the maximum PBR, and the maximum PBR zero. What is "PBR"?

Second: "line of sight angle". Wouldn't you always be 90* to your target, or close? Options for this input range from -90 to +90. How could you be -90* to your target? Obviously it doesn't mean what I'm thinking.

Third: if I use the "trajectory drift" calculator and enter zero wind, is the windage correction listed on the output my actual spin drift? If so, I get 1/2MOA at 600 yds. That sounds feasible.

Thanks for any answers provided.