on the top. for every inch it drops 2 inches of cable are pulled off the spool. this is a simple block and tackle. the are three factors that are easy to control: 1) the height to which you can raise the weight. 2) the weight (mass) you are raising and 3) the number of loops in your block and tackle. if i were to attempt this i would use a big container of water as the weight. i assume there is water available in a greenhouse. this would also be an easy way to control the fan speed. in the back of my mind i'm thinking water could be 