I have found by experience that once the temperature goes below 4 degC then I tend to miss on the low side. I have found that I can usually compensate for this by adding 5yds to the turret marking. Thats what I enjoy about FT/HFT there are some things that make you struggle for a while until you uderstand why.
Speaking to some on Sunday at Bisley they were mentioning that they were missing low. I do have a temp gauge on my scope and all day it never went above 0decC.