In designing a circuit using a transistor coupler, how should the current transfer ratio be reflected?

For the current transfer ratio (CTR), the following factors must be taken into account: Deterioration over time, LED current IF and transistor collector-emitter voltage VCE dependencies, temperature Ta dependency, and design margin. These factors shall be digitized as Dt, DIF, DVCE, DTa, and α and incorporated into the design.
For details, refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of Transistor Couplers", section 2-4. Design Example for Interface Circuit Using Transistor Coupler.