The 'long die open can' diodes, those found in *most but not all* 20x burners have a sweet spot around 420mA. Using a 3.0 Ohm resistor will give it 416.67mA. I'm assuming you use the DDL driver.
For blu-ray diodes, it's a bit more difficult. Because individual differences between each diode, they have to be measured individually. Use a pot to bring the diode just at threshold level, measure the current using a mA meter, add 10mA to it, tune the driver to this new mA figure, measure the resistance of the pot and 4 ohm resistor and that is what you need. The target current is somewhere between 35 and 45 mA (38mA average)