From MGRS grid reference which also can be found from https://www.maptools.com/tutorials/100km_square_id , the minimum column id for the first zone is 'A' and it's given when easting is smaller than 200km.
Is this mean that it is guaranteed that UTM easting value in a zone is always larger than 100km? Or, should I consider the width of first column is 200km?
Similarly, for the last column 'H' in the first zone, is it guaranteed that any UTM easting value in a zone is smaller than 900km? Or should I just handle any value larger than 800km as 'H' column?