Roughly how high are the start and finish points?
The cable should not be installed taut. This makes the tension way too high when weight is applied in the center. Professional zip line builders will typically build in about 5% sag as a rule of thumb - the percentage changes with each zip line design! The cable size, length of run, and maximum rider weight all affect the minimum required sag. So to use the rule of thumb on an unrealistically simple design, a 200' zip line that is 7' high in the middle (when loaded with a rider), should sag 5% or 10 feet. Zip Line Design This means that the end attachments are 17' high each. At this point, plan that the longer the zip line, the higher in the air you'll need to be to start. Make sure that you have a way to get that high in the air in order to ride. Furthermore, make sure that you are comfortable with climbing up to that height and being that high above the ground. Designing & building a zip line at your home can put you and others at risk of a fall.
Sometimes a starting point is known, and in these cases, the ending attachment is the plug factor during trial and error test runs (always test first with a weight in case your guess is too fast a ride to control).