Infrared Generator Circuit Calculations
Isink = Vcc/R1   seems to be the source of this value. 

Its the current that would flow if the input was grounded.

The voltage drop across R1 is (Vcc - 9.6) = 2.4V giving a value of 1.09mA through the voltage divider (R1 and R2).However this seems unnecessarily high.

If the first transistor needs 2mA through it and ( and another 1 mA to supply the second transistor) then assuming a minimum gain of 100 the base current would be 30uA so the current required through R1 and R2 would be about 10x that (to prevent load the mid point voltage too much) or 0.3mA so the 1mA current is acceptable but sightly larger than it needs to be.