Re: what lengths to cut 600\' of KM-III
What is needed, and what would be nice to have are two different things. Imagine how much heavier this rope will be when soaking wet. Is the truck too far away for a separate trip for the jug line?
It's good you are considering these factors early in the climb planning process. Imagine hanging in a tree overnight while waiting for help to arrive if you get stuck! Lots of good answers have surfaced from the group so far.
The length of rope may depend on what height your line launcher will deliver the rope setting to. If the pilot line drops over a limb that is 175' up, then a 400' line may work. Some bows will do this reliably. If you are shooting a Big Shot to 125-140' with fishing line & a 2 or 3 oz. weight, then 300' will do.
If the ground is steep, the line may fall far downhill, requiring more line. If the line has to be tied around a fat trunk as an anchor, more line may be needed.
One safety consideration is that since you bought the skinny stuff, it will literally race through your descender, especially when wet (in the rainforest). The braking effectiveness of those mechanical devices is really dependant on rope diameter.
Consider a second descender, tethered with a short lanyard just above your primary friction device.
The long ropes have lots of braking near the top of a descent, due to the weight of the ropes tail hanging down a couple hundred feet. As the descent progresses, the rope will start to have less and less grip on the descender. Near the ground, the climber receives very little assist from the weight of the rope. Small diameter lines become your enemy when you are most tired and in need of control at the end of a couple hundred feet of descent.
Please note the maximum speed stamped on the side of the Petzl Stop, for example. It says to drop less than 2 meters per second. The heat buildup can ruin that expensive line quickly. Also, with this device, it is easy to get distracted and load it onto the rope upside down, which is fatal.
One way around this slippage is to 'double down'. This means to run two strands of rope thru a figure eight.
Or, run a single line thru the figure eight twice. This is good for short runs, with the tails hanging free of the ground, so that they can untwist freely.
You'll get good at judging the stretch, to know if a line can reach the ground from the height you begin at. More ropes to select from solves lots of these types of problems.
When borrowing other climbers line in the tree, be conservative. Different types of hardware behave differently with the variety of ropes we use.
Just as you wouldn't want them to glaze or melt your line, be respectful of the investment they made when purchasing and hauling in line to the remote locations where the nice trees are located.
Be safe out there, and enjoy the great outdoors!