Global Pilot - I'm afraid I'm with WorldDC10 at being surprised at your "add 50 feet". The Decision Height is actually determined on the assumption you will briefly descend through it during a go around - in the extreme case of a Cat 3 your main wheels can touch the runway. Contrast that with a non-precision approach where the assumption (and the instrument check ride standard) is that you do NOT descend below the MDA. In that case, it is quite common to add 50 feet to make sure you don't go below MDA - but it would be most unusual to do that on a DH.
In a normal 3 degree Cat 1 ILS with limiting RVR of 550 metres, you will NOT see the lights at 250 feet - you will need to keep going to 200 feet, or you'll not get in.