Did you see the latest schematic in my edit.??
Its uses R7 to make the circuit simpler.
The -1.3V is already available in the original circuit.
The buffer is needed on the divider as the adc expects an input impedance of less than 10K
hi,
Its important to note that most rail to rail OPA's have a maximum supply voltage of +5.5V, for the this type use a 5Vreg not a 12Vreg.
Lets me know which OPA you buy.