The amount of cord you need per rib is the sum of twice the length across the rib, plus twice the maximum width of the rib, plus the distance between stitches, all times the number of stitches in each rib, plus two feet for working cord length.
That is, ((2xCapstrip Width)+(2xMax Airfoil Width)+(Stitch Distance))x(Number of Stitches)+(2 feet working cord length) = string length per rib.
On the aileron cutout ribs, this work out to roughly stretching the string and spool as far apart as my arms will go while unwinding cord five and a half times. On the main ribs, I'm stretching the cord out seven and a half times while unwinding it. The excess will get used for the blind stitches around the aileron control cables.
It's a little excess, but cord is cheap, and having to stop because I ran out of cord before I ran out of rib is really, really, really annoying. Also, I don't trust my splice knots, so I tend to rip all the stitches out on a rib and start over; I'm going to sow this so when it gets recovered in forty years, the knots are still darned good and tight as the fabric around them starts to fail. If I had lower standards, I probably would have been done yesterday instead of Maybe Wednesday - but I refuse to settle for lower standards. This plane may be heavy, but she will be durable, safe, resilient, and capable for every hand upon her yoke.
The next one can be the lightweight bush-class STOL experimental fun machine that you wear as much as you fly - this one will teach people how to fly, and bear them safely through the skies.