Set a reference voltage: using a zener diode vs using a buffer
If you are willing to do some more work, and the "offset voltage" is known and fixed, there is no need for the buffer. Instead, calculate the Thevenin equivalent to get both voltage and impedance, then subtract the Thevenin impedance from R4.

All of this, of course, assumes that the 12 volt supply stability is adequate for the threshold accuracy you need.