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 I_F and transistor collector-emitter voltage V_CE dependencies, temperature Ta dependency, and design margin. These factors shall be digitized as Dt, DI_F, DV_CE, 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.
(https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/application-note.html)