IFR - How high must batted ball travel?

[Bluefoot]

In IFR situations, how high does a batted ball have to go (at its apex) to be conisdered an Infield Fly? Obviously a line drive is not a fly ball, but lately I've had a couple of balls hit upward, but weakly, in IFR situations. There's barely enough time to get the call out of my mouth before the ball gets to the infielder because the ball is not a high-pop. But I call IFR because the purpose of the rule comes into play. If I did not call it, a quick-thinking infielder could settle under the ball, freeze the baserunners, let the ball fall to the ground, and get multiple force-outs on the play. So I call it. DOes anyone have any guidelines they use, or suggestions as how to judge the proper call?
Thanks.