I actually use 0.855 volts ... it's the difference between the LED voltage (1.57 volts) and the small transistor's base-emitter voltage. All of these voltages are somewhat temperature dependent.
The R2 must allow enough current into the transistor bases and still keep the LEDs well lit. The MJE350 for example has a hfe of "30 to 240" so it needs as 1/30th to 1/240th of the output current - a distressingly large range! My specs for the 2907A don't even specify a range for hfe, only an average value. In the original writeup, Buddha (John Camille, the first C in CCCCS or C4S) said 2mA is better than 1mA but did not discuss higher current arrangements.
I use 1/3 of the output current for currents up to 5-10mA (with a 1mA minimum), and will go as low as 1/10 of the output current where it is a burden to waste current. Hasn't failed me yet but there's no solid calculation behind that.
Note that R2 often has a high voltage across it. Resistors are specified for maximum working voltage (don't use "dielectric strength"!). In our experience, it is risky to exceed half the rated maximum, so R2 is often multiple resistors in series. The fairly new IRC GS3 series is rated 1000 volts/3 watts and I am beginning to use them for R2.
