You might also chrony at different temps and run those numbers through chairgun. If you're being completely anal about it, you could hardcopy the chairgun plot with MOA lines for various velocities and simply select the closest one to carry with you on the day of the match - I've done that and it works fairly well. Remember too, that you can pick up some extra heating and velocity by having the gun in the sun as much as possible.
I shot CO2 in a couple of matches this summer in Idaho - didn't do too badly. I've since done the HPA conversion - that's a lot better
