Have a look at this Eurobox site which also explains how IP ratings are set, (IP) itself stands for intrusion of anything,
http://www.euroboxenclosures.co.uk/IP-Ratings-Explained.html
But like I said, water can run into even the tightest sealed box through cable entries, and other places like lamps and Indicators and so on, you are going to have two entries, one for incoming mains and one low voltage out, water could get inside through these if they are not carefully chosen and sealed tightly with appropriate grommets or cable entry glands. Hence why I said drill a small 2 to 3 mm hole at the bottom most surface to drain any water that may enter, but this then destroys its IP67 rating as that means the drain hole itself will render its IP rating due to water may get in if submerged even for a short duration.
But yours is going to be mounted higher up and under a soffit, so it may not even get hit by direct rain, so long as it is air tight, all cables and glands water tight, you may not even need this drain hole.
I had a CCTV camera that was not water proof, so I housed it in a normal ABS box with no cover seal,and had a glass window aperture glued in with silicone,which made it air tight, and before i screwed the lid down I laid a bead of silicone as well, that stopped any water penetration, but after a year in use, water still manged to get inside, possibly through decaying seals or the cable entry point and the camera and the front glass plate had steamed up, couldn't see much more than bright light, took it apart and it was just the moisture inside, so i made a small hole at the bottom which helped drain any water that can enter and vent moisture.
