Donald, I've just successfully split 400ft into 100ft lengths with only a rudimentary set-up. I figured I wasn't going to do this again very often, if ever, so not worth building a fancy jig. I fixed two bolts vertically into an old enlarger baseboard from underneath, one to pop the empty 100ft core over, and the other (with a plastic biro tube loosely over it to prevent scratches) as a stop to indicate when the daughter reel was fat enough. I laid the parent reel flat alongside these, and let it revolve around the index finger of my left hand. I turned the 100ft core by inserting a square rod (such as you can find in a door handle) and twisting it. It is very well-behaved film, and doesn't fling itself all over the place in coils.
