Constant current down to about 3 volts
R1 = V - (Z1 - 0.07) / IR1 <--- IR1 = desired current...
Example: if you want 1mA on R1
R1 = 9v - (2v - 0.07v) / 0.001 = 7070Ω = 7.07kΩ
R2 = Vb - 0.78 / Ie <--- Ie = desired emitter current
Example: if you want ~10mA
1.92v - 0.78v / 0.01 = 114Ω
If you have a box of resistors and just want to check IR2 with random resistance:
IR2 = 1.92v - 0.78v / R2
I know it is more complex than that but for my cases it is close enough for anything I do
