Richard...
There are two uses for a topping lift. The primary one is to hold the boom up when the main is down. The other is to close the leech in very light air. However, most of the time when the main is up, it should not interfere with the position of the boom. Therefore a small degree of adjustability is advantageous. There are many ways to set up a topping lift. Some are attached to the aft end of the boom, run to a block at the top of the mast, down the mast and terminated in some manner (horn, clam or cam cleat). Others extend from the chickenhead down to the aft end of the boom where it is cleated in some manner (that's how I set mine up).I used 1/8" single braid rope. The length is sufficient to allow the top of the boom to mast angle to open up a few degrees past 90. I don't have the exact length...but just purchase about 6 feet more than the length of the leech...which would be about 32 feet total (the stock 26C mainsail leech is 26'2"). That will give you plenty to set it up favorably, then trim the excess. If you choose to run it down the mast, you'll need about 70 feet.