Hi Andrew,

Here is what you do:

Put a 0.1 ohm 1 Watt resistor in series with your LEDs between them and
ground.

Use an LM2904 op amp to amplify the voltage across the 0.1 ohm resistor
such that the LM2904 outputs 1.2 VDC when the desired current is
flowing through the resistor. The gain resistors you need is something
you should be able to figure out.

The output of the LM2905 op amp goes to the Feedback input of the
LM2576.

Best Regards,

Dean Wilkinson