Hey Sean good question. Not all ammo belts are created equal. With that said my ultimate goal was to leave as little return edge as possible yet not cutting into the box return edge themselves as seen here in the RS reference pic.
In my case this left me 2-3mm clearance of the boxes on each side of the belt. This just so happen to leave me almost exactly 4" total width when finished.
If you have a ammo belt total width of 4" (101.6mm), removing 3mm on each side of the belt sill leaves the belt width at 3-3/4" (95.6mm) or 3/8" (9.5mm) over on each side of the cloth belt (granted the cloth belt you have is 3").
Bottom line, trimming the belt 3mm on each side shouldn't sacrifice the overall look of the belt especially as in the area where you will taper it in the corners to meet the cloth belt edge.
Hope this answers your question.