Creating an adjustable output regulator, in LTspice
The output voltage of a 7805 is 5V relative to pin 2 - if pin 2 is raised a volt above ground then the output voltage is 6 volts relative to ground. Because there will always be a small (and fairly constant) current flowing to ground from pin 2 (usually about 4mA =\$I_Q\$), the offset voltage on pin 2 can be 1V (if R is 250 ohms) or if you wish to do it more accurately you can use two resistors, R1 and R2 forming a potential divider from the output to ground. This uses the more complex formula in your question and \$I_Q\$ is as stated above i.e. about 4mA. \$V_{XX}\$ is the output voltage if pin 2 were grounded i.e. 5V for a 7805.

However, your circuit uses an op-amp to raise pin 2 so whatever voltage is at the output of the op-amp adds to the 5V on the output just as if you inserted one resistor in the common line.