How to estimate the overall CTR of this optocoupler from its test conditions
The datasheet says that at IF = 7.5 mA, the CTR is typically about at least 95 % of that at 10 mA, and that between −30 °C and 30 °C, the CTR is typically about at least 90 % of that at 25 °C:


Of course, for different IF and VCE values, the relationship might be different.
Vishay's CNY17 datasheet is more informative:


Your circuit wants to pull the output as high as possible, so you do not care about the CTR when VCE = 12 V, but when VCE ≈ 0.4 V, i.e., when the phototransistor is saturated. Like with 'normal' BJTs, you need more base current (or here: more IF) to make the output saturate.

So, the worst-case unsaturated output current is 7.5 mA × 160 % × 95 % × 90 % = 10.26 mA. Even with a safety factor, this means that an output current of 5.5 mA will saturate the transistor properly.