There is really no reason to try to measure it before yo ustart unless you think you do not have enough. The 7 times rule of thumb is a guideline, but there are many variables (how far you space your slots, how far the slots are from the edge, etc). Pull out as much as your arms are long, about 5'-6' and go. You will likely have one splice in it before you are done. DO NOT try to do it with one piece. The lace will be worn out and very stretched before you are done.
Aaron