Sorry, I got sidetracked. In "normal" operatoin, the reference voltage is derived from the rail with a trio of resistors that divide the rail to VCC*2/3 for the "threshold" and to VCC*1/3 for the trigger voltage. If you use the control pin, you replace VCC at the threshold with your control voltage, Vc. And Vc/2 for the trigger. So, suppose you use 3.3V divided to 2/3 for the control pin, or 2.2V. Then the trigger will be 3.3V*2/3*1/2, or 3.3V*1/3, which is 1.1V. The way I would do it is to use a voltage divider for the 2/3, say a 33K and 66K ( or whatever standard resistors are the closest). Connect that to a CE configured transistor, and the output to the control pin. Now, you have 3.3*2/3 connected to the threshold, and all the math above holds.