Put the CMOS gate/buffer just ahead of the RC. So the TTL signal probably is 0V and 4V the CMOS buffer 0V/5V.
I tied 2 and 6 together and injected the RC signal there.
The 1/3 and 2/3 voltage points are set by the three resistors from 8 to 1. I don't remember but I think the resistors are 10k. ????
Connect a 1k from 8 to 5 and the "TRIG" voltage will be just below 50% and "THERS" will be about 95%.
***I need to look at the data sheet to see if the inputs will work that close to VCC.***
View attachment 108565
Another option is to add some resistors to 6 and some to 2 to shift the voltage up or down.