Sideoiler,
Worrying about the ambient temperature will not be a concern if you're finding only a .002 difference between your hot lash and your cold lash settings. Depending upon your head, valve, block, rocker arm combination, the difference between hot and cold lash can be significantly higher. Some people may find a difference of up to .025 or so. In those instances, a difference of 40 degrees in your "cold temperature" can be significant.
Here's just one example of someone encountering a difference of more than .018.
http://www.network54.com/Forum/74182/thread/1146187108/
Chris