...The correlation between the two varies with rain intensity, direction, windspeed and direction. Usually, the CoCoRaHS gauge reads higher than the ISS gauge. I occasionally adjust the daily setting in WeatherCat if the difference is significant. I generally trust the CoCoRaHS gauge over the ISS gauge.
That's been my experience as well....the nearby (30 feet away) CoCoRaHS gauge typically reads higher.
I'd add one more variable to your list which seems to make a difference at my site...the number of rainfall periods during the past 24-hour period.
Sometimes my Davis gauge is right on the button (compared to the CoCoRaHS manual gauge) and other times it's off by two or three hundredths. I've always figured that the rain likely stopped just before the bucket tipped the last time so that accounts for a hundredth. And then if there are multiple rainfall periods during the day with evaporation taking place out of the bucket between showers, that could account for some additional error. Plus, if the wind direction is such that the mounting pole partially shields the Davis collector, I get even more error.
I have a friend in another state with a WeatherHawk 600-series station and he tells me the raindrop-counting solid state sensor correlates nearly 100% with his nearby CoCoRaHS gauge (both gauges well outside the influence of any obstacles).