RE: How do you choose right arrow spine??
In general, you can use the arrow manufacturers tables to get you in the ballpark, then use bareshaft tuning to determine if your are stiff or weak and adjust your draw weight accordingly. The Carbon Express sight has a great worksheet for determining "Adjusted Draw Weight".
As an alternative, you can leave your draw weight alone and start with your arrows a little on the long side. Bareshaft tune and then continually cut the arrows down in 1/4" increments until you achieve good results with the bareshaft tune. BTW: this is a royal PITA if you don't have your own arrow saw.
Or as Ausie-guy said, you can use one of the computer programs On Target, Archers Advantage or The Archery Program, and work out the number to give you a near perfectly spined arrow with your particular setup.