You don't need a chronograph. The 'speed ring' ring setting is nothing more than the variable magnification changer, it generally doesn't correspond exactly to arrow speed anyway.
Here's how to set it. Set the speed say for 300 fps. Next sight in the bow at 20 yards using the crosshairs. Now back up to 30 yards and shoot at the same point by using the next aiming point down (which elevates the bow slightly). If the arrow hits low DECREASE the speed setting; if it hit's high INCREASE it. What you're really doing is, with more or less magnification, changing the distance between the aiming points. Then try the next aiming point down at 40 yrds. Like as not it won't correspond exactly but you'll have a good idea were the arrow hits. You can play with it a little by fudging a bit on the 30 yard point of impact (a bit hi) to hit on at 40 OR you can sight the crosshairs at, say, 25 yards and see how 30 and 40 come out. It's a trial and error process, whose objective is to get the aiming points as close as possible to the actual trajectory of your arrow.
Note: Once you sight the crosshairs in, the point of impact won't change - at the distance you sighted in for - when you change the speed setting; however, the other aiming points will.
20" arrows are OK but watch out for the broadhead sticking out past the bow, and they will be a bit heavier, so slower.
Good luck, don't hesitate to ask for clarification if needed