It'll be be necessary to use the minimum CTR given (50%) for proper circuit operation. Ensure that the upper limit doesn't give you a problem.
It's important to note that many semiconductors have rather broad specs. Current gain for a small signal transistor, for example, may vary as widely as 150 to 900. You can either buy the broad spec one or in many cases a selected part may prove beneficial to you. Loads of Pro-Electron transistors e.g. BC548 for example have A. B and C gain ranks and many Japanese devices have 'colour' gain ranks, e.g ORG (orange), GRN (green), BLU (blue!). Similarly some power devices have A, B, C, even D voltage ranks.
In the case of an optocoupler you're subject to 2 variables. The luminous flux of the emitter diode and the sensitivity of the receptor diode (junction). So the apparently wide CTR of 12:1 is only a difference of 3.5:1 for each part of the device.
In volume manufacture it's often possible (if you need) to get parts selected to your spec although it's inadvisable to rely on this.