Umm, you've got the formula! FPS = Feet / Seconds = Distance / Time.

At 310 fps it will take .1935 seconds to hit a target 20 yards(60 ft) away.

60/310=.1935 seconds

60/280=.2143 seconds

The speed of sound is approx. 761.21 miles per hour. Translated is equates 1116.44 fps.

The sound of your bow/arrow will reach the deer as follows:

60/1116.44=.0537 seconds

The speed of sound is roughly 3.60 times faster than the 310 fps arrow or 3.99 times faster than the 280 fps arrow. Now, all this is based on static numbers, but your velocities of your arrows will degrade over distance traveled....thus making all the above numbers completely irrelevant. But, the theory and premise are there for you.