I understood that cloud base is not the main factor in determinining approach limits, but that it is the visibility. The problem with the IMC combo of 1800m and 400/500 feet is that on a 3 degree slope, at 500 feet, you'll still be almost two miles from touchdown in a vis of a mile, so at the limits, you may end up throwing away a perfectly legal approach simply because you can't see the runway until you get a little further along.
In practise, it may be better to confirm sight of the ground at 500' then continue down the GS a little further until the runway comes into sight. Of course a decent CAT2/3 lighting system at the airfield helps greatly as the approach lights will easily be seen at 500' even if the runway isn't.
I've taken a couple of PPL/IMC mates in the sim and frozen it right on their limits to show them how murky it still looks at 500' but that within another 10-20 seconds on the approach, the whole picture changes and the warm fuzzy feeling follows.