Archery Coues

Hey everyone i am new to the Coues deer hunting scene and have a question. I plan on archery hunting on the 23rd and need to know how much can a buck drop at 20 yards when he hears me release. That being said on a calm buck. My cousin says around 4 inches. My hunting set up is right around 300fps. Just want to make sure if I get a chance with my bow that i dont shoot over him if he drops. Thanks in advance.