It would help if you provide a link to the specifications for your motor and encoder AND provide a diagram of how everything is connected. A photo of a clear pencil sketch will be fine.
Would it be better to trigger the interrupt with a RISING pulse? I don't know if your present code will give several interrupts while the encoder output remains HIGH?
At first glance your code looks more suited to a stepper motor than a DC motor. I don't know whether it is possible to get a DC motor to move exactly to a particular step as there would be nothing to hold it there.
Wouldn't it be much easier to do encoder1Pos += 12; and not bother with multiplication.
...R