This is a very rough rule and ignores the complex physics and wind but if you look down from where the highest portion of your body is you should only see fender.
Of course the real issue is that once you start to really shorten a fender the water that is ejected onto it will drip off onto the tire and if short enough into the cassette. This will dramatically increase the amount of grit and dirt that your drive train is exposed to.
I live in an area where I have to cross a ship canal where I have to walk the bike on one wheel and I also work in a place with vertical hanging bike locks. Both of these require me to have a bit shorter of a fender and mine is cut at about the 10 o'clock position. I do have a planet bike "hardcore" fender that has a bit of a flap on the back so the drip edge is behind the tire. The rooster tail at 18 mph with no wind seems to be about two bike lengths long or below safe following distance in wet conditions IMHO.
Look for pictures of "race blades" IMHO that is as short as I would go if you use it in the rain often. The angle seems to be typically in line with the top of the chain stays. All of this is just opinion, but in my opinion, rain is just wet but road grime filled water just plain nasty.
Note: I have a "cascadia" front fender because dry feet rule but this would not be "ultra-lightweight"