I can't remember the exact ratio, but when I had a room skimmed a few years ago I was told to just buy the cheapest white paint and water it down with whatever ratio they suggested at the time and apply several coats before decorating properly. The only issue I've had is paint flaking off behind a radiator and I'm not really sure how to stop it happening again in the future when I paint again behind the radiator.
did you put the radiator back on quite quickly and then use it? force-drying is not good for paint