According to my calculations a 14mm would be .55cal and a 15mm would be .59cal
I'd go with the 15mm
I use a .750 punch for my 12ga (.729? bore) wads. I also find the method of 2 cardboard (I use old milk cartons) wads over the powder and 1 cardboard wad over the shot works fine for me.
Only one type wad to carry in the pouch meets my K.I.S.S. methods of loading criteria.