Blodg,
I believe temperature has more of an impact than a lot of people think as well. I use my chrony in doors with a light source and generally I get within 1 ft/sec on the same bow/arrow combo on different days. However, a friend brought a bow in one day for me to measure it (it had been overnight in his truck at 25 degrees) and after measuring a few times we noticed that the fisrt few were 2 to 3 feet/sec faster. This certainly wasn't a controlled test and I haven't tried to duplicate it but you might want to keep this in mind.