Glad to help. We don't want to deprive students of the opportunity to learn.....
OK, 42V/12=3.5V, so 700mA is close to the current you'll get. Remember that the Vf is never exact, and has a negative temperature coefficient (Vf gets lower as the LED gets hot), so the 1A current limit is still important.
For the LCD back light, a resistor is all you need. 0.8V / 0.3A = 2.6 ohms. You aren't obligated to supply the maximum rated current to the back light. I would go with 4.7 ohms or higher as long as it's bright enough.