Thanks joeyd999.Using a generic voltage regulator as a voltage reference, in this kind of application, is not a good solution, adjustable or not. The LM317 has 1% time and temp stability. This is equivalent 100mV potential drift at 5V (ignoring the tempco and stability of the resistors and adjustment pot.).
By comparison, an LM4041 adjustable reference has about 100ppm time and temperature stability, or 100 times better! Remember, he is trying to qualify parts based on a few tens of millivolts difference.
During that time that I just thought the LM7805 may not reach to 5.00 or something like that, using LM317 maybe can do that, so I didn't have other thinking, sometimes I will waiting for the response from op, after you mentioned that, yes, using generic voltage regulator is not a good idea, normally I will using Tl431 for Vref, do you think is it ok for that?