That's quite simple... say on power-up a Button is held for 5 Seconds... the LCD Displays...

"Adjust for 90v"
"And Press SET"

When the Button is pressed, the ADC WORD value (containing the 90.0v reading) is then saved as a HighBYTE and LowBYTE component into two seperate EEPROM locations (since each EEPROM location can only save a BYTE).

When you power-up normally, rather than jumping into the Calibration routine (described above), your program instead loads the CAL value from EEPROM, and you would use the CAL value in your math with a modified routine, rather than the 1023 value routine previously exampled earlier in this thread...

0.1% Resistors are probably cheaper than paying the Labour to run through a CAL routine at manufacture. One of your divider Resistors could instead be a multi-turn high-stab precision POT (so you could also Calibrate for 90.0v using that method instead).