For fun I just looked up 5 and 6 mill cordage.

5 mill is rated at 5Kilonewtons (about 1,000 pounds)
6 mill cord is rated at 6.8 kilonewtons (about 1,400 pounds)

A kilonewton is 224.8 For simplicity and safty I always calculate a kilonewton at 220 pounds.

Before we go to much further, we need to also look at the knots in the system.

the average knot takes aobut 30% of the strength of the rope. The best knot is the one you can avoid using, seriously! here is an exaple.

lets say you have to rappel a short 50 foot drop.
As luck would have it you have your handy-dandy 120 foot long 6 mill cord, a couple of locking Carabiners, a sit harness and a figure 8 device. The area is wooded and there is a solid 10inch diamater pine tree.

the best way to attach to that tree is to take wrap the rope around the tree at about head height, keeping the midle of the rope on the back side of the tree. simply doing a gian U shape with the legs of the "U" facing down. this way you can recover your rope when you get to the bottom.

If you fancier, wrap the tree twice with the center of the rope pointing towrds the direction you are going. this way there will be one full wrap around the tree. But be aware that you wil have to pull very hard to get the rope back.

Now belive it or not, by simply wraping the rope around the tree you keep 100% of the ropes strength, untill you hit you belay device. then you start to loose about 30%. Simple math says that 30% of 1,400 pounds is about 420 pounds so you are rappeling with a remaing 1,000 pounds of strength. Since this is my fantisy we will assume my weight wich is *clears throat* 280 so I would be just under ad 4:1 safty ratio. NOT the best, but safer than flying on a plane (comercial air lines are at 1:1 last I knew) FYI, when doing SAR we like to see a 10:1 or 15:1 ratio.