It is not just height of the ball that we have to consider for IFR. We also need to determine "easily catchable by an infielder." To develop an equation for IFR, let's consider the following information:
A = Players age
P = Player height
H = Height of ball
D = Distance (player movement when ball is at apex)
We could the use the following formula to aid our IFR decisions.
IFR FACTOR = H/(A - P - D)
As this factor increases, it is less likely that the fielder will be able to catch the ball. For instance with a ball hit 20 feet high an 8y/o who is 4 feet tall and doesn't have to move has a good chance to catch the ball (factor = 5). This same player having to move 3 feet increases his factor to 20 and will probably have a difficult time catching the ball.
Remember - you will need to make this calculation whill the ball is at the top of its arc
-Kono