I think he is referring to the little mph they always post during the Little League World Series. So if the kid is throwing 73, thats equivalent to 99 mph from 60. They never actually stated what the math behind that is. I would assume they are going on the time from release to plate.
I don't remember the actual equation that you use. I ran it through a proportion (60xMPH thrown--> that total divided by 46). So 54 mph from 46 feet would equate to 70 mph from 60. That is in terms of time. Speed would not change.
Hopefully someone can tell if that math is correct.