Consider creating more than one range for your voltmeter similar to store-bought meters.
For a range 0-100V you'd have 0.1V accuracy or 0.1 percent accuracy, whichever you want.
Or by subtracting a certain amount you get an expanded scale meter. Therefore to measure 900 to 1000 V, first subtract 900 using a series zener diode. (In practice you'll scale it down with resistive divider.) This effectively results in 0-100 V to be measured, with 0.1V accuracy or 0.1 percent accuracy.