With clear film, I get 15-20 degrees of gain on cloudy days, and 50 or more on sunny days. My greenhouse has the clear film, but my high tunnel of tomatoes has opaque film. It does not get quite as hot, and the tomatoes don't get sun scald as badly.

Even on the cloudiest days there is some gain. About like Robbie states. Mine even gains about 6F on heavily overcast nights. This is with IR and woven poly.

I assume the gain is better in April than December?

My brand new greehouse has at least a 10-15 degree gain during the day on a very overcast day in oregon. But on a sunny day its much more like 20-40 degrees. during the night it stays at least 6 degrees warmer than the outside temp. which is good to keep tomatoes from getting cold stressed from 40 or bellow nights. i have a review of the green house i have on my youtube channel if you would like to take a look.

Here is a link that might be useful: TheItalian Garden