when i made that circuit to test it, i used a potentiometer.Either its a different symbol for the same thing, or I used the wrong part
You can connect a potentiometer so that it works like a rheostat
Hopefully its just me , not doing the calculations the right way.
Remember you can put resistors in series or parallel to get closer to the value you want.
Don't get too concerned about the exact value of the series resistor. It's value was determined from the nominal value of the reference voltage which may or may not be 1.24 volts and a desired LED current that we pulled out of a hat.Don