-300 mV Voltage reference
If you have a negative power rail then use a shunt voltage reference - basically it's a precision zener diode so it can work on either supply rail. On the negative rail the cathode connects to ground and a resistor feeds the anode from the rail.

Here is one from LT - it's the LT1389 and the first one I found. It's a 1.25V reference and you can use two resistors to divide this down to 300mV. You need to feed it with a minimum of 0.6 uA but because you'll have a potential divider in parallel you'll need to feed the reference and potential divider with a slightly bigger current. You might as well use 1 mA (the device is rated up to 20 mA anyway).