I lost about an avg. of 75fps on my loads from a mild day to a cool day. Maybe 10 celsius diference. This was with several loads with this powder and more than a few shots over the chrony.
Is there some sort of table for different powders and expected loss of velocity with cooler temps?

I have never noticed a 75 fps. difference in H4831 or H4831SC out of my .308 Norma Mag., that is why I switched to an extreme powder. What primers are you using? Since I am using Win. or CCI LRM primers I don't switch but I have read many times about people switching to a magnum primer from a standard primer for cold weather. I don't know whether this would be safe in your gun or not though.

There's a guy who is in the middle of an experiment using a strain gauge and a thermo couple for barrel temps. He's come up with numbers indicating something like 7 fps per degree Fahrenheit difference. He'll be testing further with low ambient temperatures once the weather turns. He found the same sensitivity in the Hodgdon Extreme series as with any other propellant shooting in the 70� range.

I'm pretty sure above that temperature ALL propellants increase pressure. At least that has been my experience. Below that, and on down to freezing, I've chronographed some of the Extreme line versus some others, like RL22, and they don't seem to lose as much velocity but my "tests" haven't been very scientific. Just observations while recording the ambient temperatures while shooting at the range in various temperatures. As a result I believe ALL powders are significantly affected by heat above 70� F. I believe there is some merit to the Hodgdon line in temperatures near freezing.