I would suspect that there are a lot of factors that influence the formula. CaDad pointed out an important one when he mentioned the crow hop. Another thing we’d need to know is what type of drag is calculated into the formula. Do these numbers assume a large drag factor like a knuckle ball will produce or do they already assume a 4 seamed, 6-12, tight rotating fastball?
Obviously wind can play a large factor when throwing that far but if we really want to get real scientific, you need to know other factors such as altitude, humidity, etc. At that difference the condition of the ball can even make a measurable difference.
The bottom line is that it appears that going off the chart has plenty of margin for error but when Coach May’s son or anyone throws a non-wind aided 310 feet, I think it’s safe to say with reasonable certainty that he can throw mid-eighties for sure and possibly higher; outstanding for a kid his age.
A side note: In the olden days before radar guns, teams used to hold contests to see who could throw the furthest. I recall some throws were of incredible distance and must have had a decent wind behind them although they never mentioned it. It sure makes for some tall stories looking back at the distances. I forgot where on the web I seen this but I thought some throws were recorded at 400 feet or more.