I would not use the markings from RS. Their ammo belt was not cast from their original armour so it is a custom sculpt and cannot be compared to any original belt. There are things on the RS armour that are not original cast and those that are have been altered slightly for whatever reason, so do not take everything as truly screen accurate.
Any information I post is to help people build their armour like the originals and this information has been gathered over many years. I keep reading about the RS armour and the issues people have with their kits and how that is how it must be because it was cast from an original. The truth is it was cast badly, parts altered and parts custom made, so the majority of it is not a direct, unaltered cast. I am not trying to bash RS but they are not trooper fans who have spent years studying original armour, they are just two guys who make armour.
My advice for your belt trimming is to cut off 40mm from the ends of your last ammo blocks and then mark top and bottom corners 10mm from the ends and sides 12mm and cut the ends. They corners are not 45 degrees if you look at any original belt.