Instead of messing around with precision resistors and worrying about temperature tracking, etc, why not use a "rail splitter" IC like this to get your half voltage? It has a temperature compensated precision trimmed voltage divider built in. Just feed your input voltage into the input and connect the ADC to the output.

If the voltage range you are measuring at the output of the divider is between about 2 and 20 volts, you could use a "rail splitter" IC like this instead. It has an onboard precision-trimmed and temperature compensated resistor divider. The input has to be within the compliance range of the chip, however, hence the output voltage range restriction.