Calculating resistor values for a Schmitt Trigger and LDR circuit
I can tell you a general procedure that should get you close.

First, find a value of R1 that gives you 6V across R1 when you have 250 lux. Set Vref at that value, 6V. Now we assume that you want the op amp output to go high when the R1 voltage rises above 6.5V and go low when the output voltage falls below 5.5V.

Assume that the output is high (12V) and you want it to go low when the voltage falls below 5.5V. Assume that R2 and R3 are much larger than R1, say 50X bigger so that they don't have much influence on the voltage across R1. Now, R2 and R3 form a voltage divider between the 12V op amp output voltage and the 5.5V across R1. You want the voltage at the non-inverting op amp input to be 6.0V at this point, so the op amp output will fall if the R1 voltage gets any lower. So you need 0.5V across R3 and 6.0V across R2. They have the same current flowing through them so the voltage ratio is also the resistor ratio.

Now assume that the op amp output is low (0V) and you want the voltage at the inverting input to be 6.0V when the voltage across R1 is 6.5V, so that any further increase in the voltage across R1 will cause the op amp output to go high. Once again you have 0.5V across R3 and 6.0V across R2 so the same resistor ratio works in this case.

So, choose resistor values so that R2 is 12X larger than R3 and so that both are much larger than R1. This is a rough, back-of-the envelope design but you can experiment and tweak the values as you like. The important thing is to set Vref at one half of the supply voltage, then the ratio of R3 and R2 will set the amount of hysteresis.