I think you will find that adding 50 feet to the minima is for PEC. Pressure Error Correction. That is, certaing aircraft altimeters are graded to a specific standard. In your aircraft flight manual it will tell you what the
PEC is for your aircraft. On the ILS particualrly when you are so low to the ground, the law asks that you add the PEC to your minimums so as to not break your minimums with a dodgy altimeter.
It sounds as though you are asking a PEC question in which you should consult the flight manual. I'll dig out my YWE chart and have a look.
It has been a while since I instructed so forgive me if i'm completely off the track