JBM questions

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.

PBR= point blank range. Not sure about JBM, but usually the point which keeps the bullet no more than 3" above and 3" below line of sight on its way to maximum point blank range.
line of sight angle= you could be shooting up or down hill. Enter the angle above or below horizontal. If you're shooting uphill 10 degrees, enter +10
you answered question 3 yourself.

Thanks, now it makes sense to me. It's been a long day:stupid: