I am with Moss in not cutting a piece of your climbimg rope for this purpose. A couple of split tails is a good addition to your climbing gear but there are other alternatives to get one that will do the job and save your long rope in one piece.
Sherill offers several split tail ropes at a reasonable price (cheaper than cutting your new climbing rope).
Here is a _link_
http://gear.sherrilltree.com/iwwidb.pvx?;multi_item_submitAnother cheaper alternative is buy some 10 feet of accessory cord (8 mm works well for me) and tie your own split tail using one of the regular knots appropriate for the task and that set up can be very low cost anything from around 50-90cents per foot and higher up as fancy as you want to get.
Look at some alternatives below.
Prusiks and accessory cords
http://gear.sherrilltree.com/iwwidb.pvx?;multi_item_submitIf you are planning to attend the 2008 Rendezvous in Atlanta in about four of weeks Some of the gear manufactures will be there offering climbing supplies at discounted rates (sometimes they do that).
Looking forward to the Rendezvous!
I bought 150' of Prolite 24 7/16ths. So, should I cut off 4' of my climbing rope to use as a Split Line?A "4 feet piece" is not long enough for a split tail. You will need around 8 to 10 feet piece for a good working split tail so that you can tie the needed knots, anything shorter is just wasted rope. It will not work for anything.