That is what the regulated voltage is on the LM317. (between the sense pins)
It is actually a voltage regulator. We use it to regulate current, which it does by regulating the voltage very precisely to 1.25 volts.
To figure current...
1.25/ohms of resistance = current.
Example:
1.25/10 ohms = 125mA's
1.25/5 ohms = 250mA's
And so on...
The LM317 will take up to 37 volts supply. For a red build, you will want to supply the driver with more than 6 volts. Which is why you would use 2 X 3.6 volt batteries. Because 2 X 3.0 volt batteries will start to drop below 6 volts very soon. Then the LM317 will 'drop out' and not regulate the current.
For a blu-ray build using a DDL driver like this, you would want 9 to 12 volts supply...
Jay