That depends upon how you want to rig...
it. Do you want to control the lift from the end of the boom, through a line stopper into the cockpit, or a cleat at the boom/mast joint? The minimum would be the length of your mast times three. That should get you from the end of the boom to the top of the mast and into the cockpit with some left over. Other 28.5 owners may have a more precise number for you.Terry