To chop your rope, first wrap the spot you intend to cut with some basic finger tape. Slice through the rope and tape and then burn the exposed end with a lighter so that the tape, core, and sheath melt together. You can write on the tape the new length of your rope, but be sure to use a rope-specific marker, as regular ones can damage the cord. Also, consider marking a new middle point, as that will have shifted (or just cut both ends).