I am trying to think whether you can do some post-processing to incorporate the constant. Here is the idea: if you set the flow rate to simply 1.0, then the flow value you are getting is essentially flow sensor click count. From that can you use your formula to calculate the actual flow volume? After all, the flow value you are getting currently is just the flow sensor click count multiplied by the flow rate. So you can as well set the flow rate to 1 and do some calculation afterwards.