You use 20 yards and 60 yards not fifty. It doesn't really matter which pin you use to make the marks. Since you are running 4 pins I would sight those pins in first at your preferred distances before setting the slider up. Once you have your pin gap correct on those 4 you can do everything with gang adjust.
Put your top pin all the way up in the housing and sight it in, use the wheel don't move the pin. This will give you maximum distance to use the slider. Then get your pin gap for the other three. Using a 4 pin I would designate the bottom pin as the slider. Using the gang adjust wheel sight the bottom pin in at 20 yards and make a mark. Then again using the gang adjust wheel move the bottom pin down and sight it in at 60 yards and make a mark. Use the enclosed ruler to measure the marks and get your tape.