biggest issue is the timing, would need to stopwatch it (and then human error on stopwatch gets involved, because timer on video is not precise enough (ie gonna need to go down to tenths maybe hundreds of a second).40 feet (or correct distance from pitcher’s plate to home plate) divided by the time from pitcher’s release to the catcher’s catch = speed in feet per second.
One foot per second = approximately .682 miles per hour.
Multiply the speed in fps by .681 to convert to mph.
Note this is still approximate since the pitcher does not actually release from 40 feet and the catcher does not actually catch the ball at 40 feet from the release.