If I want to cut a set in half, I just loop around a doorknob at the end of a hallway. Start with set in left hand, take right hand and hold one end, hold a bit of tension and walk away, and let the set gently unravel, when you get to 50/50, keep lines taught, walk towards door handle and cut the middle. Make a mark where that halfway point is with a pencil somewhere. Measure that, that's normally 20.5 feet or just over. If you don't feel comfy with the 3.5 times your wingspan, take the reel, if you don't have a starting clamp, use a clothes pin, even a piece of tape, secure the end piece to door handle, walk down the hall with reel to measured off spot and cut. Just make sure to hold onto the string so it does not fall and get tangled. The door handle method is the fastest way, for me, to make sure I'm cutting a set in half.
For a reel, the 3.5 wingspan works fastest. When in doubt, cut a bit more. I don't worry about that extra bit that could be one more string job. Get burned once and it's a waste of time. But you can rescue most too short of string situations with a starting clamp and and old piece of string. But that's getting slightly into a slightly more advanced technique.
Lots of light helps too, my stand is in a weird spot and I don't want a lamp there so I wear a headlamp.
