A bit confused. I thought this was a line for checking / allowing for cargo you plan to pick up during a mission-either internal or sling load. Yes, it adds it in at TO weight but that gives you a worst case scenario. If you are below Max at TO, you are good to go. If above max at TO with the planned cargo, then you need to estimate fuel burn, enter that as your fuel load and try again. If still above max, reduce fuel at TO, re-estimate fuel burn, stick in the new numbers and see what happens. Wash, rinse, repeat.
That said, I will admit to never really using it…if you ever witnessed my sling load “skills” you would know why …but that is what I thought PLANNING CARGO was for.
So now you are making us use something called “math” to figure out weights…you evil, evil, people you!