Another method would be to use one of those precision 3 terminal 2.5vdc voltage reference chips. Just wire it to one of the analog input pins and use that value as a 'correcting source'.
As you say, that would take an analog pin and i'm out of them already(big project, pin constrained in all sides).
And there would be no need to do that if i have the bandgap correcting the input
i'll have to make some tests/exalibrations once the device is more than a jumbled prototype, once i finish building a prototype of the analog part that needs the precision readings i'll do some tests.
Maybe i won't even need the code when i finish building and callibrating(but i doubt it) since i'm having at least...:
+-18Vdc opamp supply(probably 7818+7918)
+12V fan line (simple 7812)
+5v precision DAC supply
+5V "general line" for AVR, LCD+logic, DAC logic, LEDs, buzzer, digitemp sensor
the 5V precision will be derived from the 18V high rail, the general from the rectified bridge output of the bridge.
so yeah, a mess of different rails all over the place 