If you serve flat (little or no kick) and your serve bounces once in the box then hits towards the bottom of a fence that is 21 feet behind the baseline, what the minimum speed it needs to be traveling?
A common rule of thumb (not an exact science),
for a normal flat 1st serve attempt that hits in the svc box,
then the 21' away back fence "solidly" on one bounce on hard court,
(opposed to barely making it on the way down)
will normally be in the 100 mph "range" ,
(possibly slightly less)
as compared to the ATP system of measuring the speed right off the racket face.
( it will not be 100 mph as measured by a speed trac on the wrong side of the net unless it is well over 100)
-had to really spell it out, as there are some who think for every exception they can come up with, somehow that affects a rule of thumb.