When the sail is up, the topping lift should be set so that it is completely slack. The topping lift has no effect on sail shape unless it is left too tightly set up and then the effect is negative. Depending on how your mainsail is shaped leaving the topping lift set to one single position can often result in the end of the boom sagging down into the cockpit a little more than most people prefer so a lot of boats are set up with adjustable topping lifts that are eased when the boat is sailing and tightened just before the sail is dropped. Topping lifts on a small boat can be small diameter (1/4") dacron double braid. You can also use 1/8" SS wire with nicropress fittings at both ends, cut short of the full length, with a rope tail.