I did something similar for a temporary drop-below-original test with DS18B20. I think this simply involved copying a command in the loop to the end of the setup
tempC = sensors.getTempC (deviceAddress);
and adding
flag=tempC
and the end of the loop was
if (tempC<flag)
{
serial.println(" Temp C < flag");
}
If you want multiple cases, I guess it is simply a matter of adding them - flag2 = TempC + 16.16 etc., and writing an if section as needed. No arrays needed.
I recognise that you are currently only using one DS18B20 but, FWIW, I submit you would be better off determining the device address in advance and using the simpler Hacktronics approach.