It's the most pointless column in the log book. Where I work we simply log a fixed amount per sector as PF. In NZ the rules state to log time as IF if it is spent "navigating solely by reference to instruments" or something similar. Which is from takeoff to shortly before touch down on every flight. There is no reference to cloud. So should we be logging the whole flight? But then there is this 10% rule of thumb. Where did that come from? I think a more accurate column would be a "IFR flight plan" one.