In a way, there isn't a useful answer. That's because the Burning Software will determine what IT can use. I can add and substract so correctly, but if the Burning Software disagrees by any tiny percentage - IT WINS. So MY determinations aren't too valuable.
When I do mass backups, I really DO like to 'calculate' how much I can put on each disk, but after a while, I'm dragging files onto the "Target Disk" and watching its Capacity Bar get filled. When it's over the limit, then I chop some files out. My calcs, my prayers, rants, etc, just won't change that software's mind. It acts like it just doesn't care what MY calcuations are! Drat.